Enhance your assassination techniques

Assassination feats can also be completed in contract mode.
If you're struggling with targets ignoring toilets (Hold My Hair challenge) or you don't have an electrocution phone for easy accident - look up a contract in your preferred location and filter by kill condition.

Specific kill conditions such as chandelier or coconut drop can be done by pacifying your targets before attempting them. This way you avoid having to time your shot. To simplify the challenge - start the map on casual difficulty - you can pacify or eliminate any guards who are in your way with small risk.
Same goes for oil drum fires. You can drag knocked out NPCs to oil drums and create a fire accident without having to manipulate AI to go there. You can puncture oil drum without screwdriver by shooting it once. Your next shot will ignite the oil.
Lethal poison injection/consumed poison kills, gas container explosions (like one in the Haven jetski hut) and falling accidents don't ruin SA rating when bodies are found as long as no one sees you inject/push/shoot.
Targets can see you perform illegal actions and you keep your SA rating when you eliminate them.

Target saw 47 trespass and SA indicator turned red.It returned to green after target died.
Note: If a contract has "don't get spotted" condition, you still fail that even though SA rating is not ruined. Dropping a gun on the floor separates guards from targets and helps remove guards from their posts. Multiple guns can clear out several guards at the same time.

Entering cover behind a wall near open doors (or behind half-covers like a stack of wooden pallets) helps subdue people walking towards you without you being compromised. All you need to do is keep pressing your subdue button and 47 will grab an NPC and choke them out.
This won't compromise you unless you press 'E' and some NPC hears neck cracking sound. They will investigate this and will find the body.

If you need to lure someone into a room, but there is nowhere for you to hide (no closets or garbage containers) or the only hiding spot is too close to your distraction, simply stand near a wall next to the hinge side of the door. This way when the door opens you will be hidden behind it and NPC will not spot you with their peripheral vision.
You can then close the door and pacify unsuspecting NPC.

Note: Does not work with sliding doors for obvious reasons.

Briefcase allows carrying illegal items without attracting attention. You can't pass frisk with an illegal item in a briefcase, so place it on the ground, pass a frisk then pick it up and guards won't question you or make you pass frisk again.

Briefcases also allow you to carry things out without alerting everyone (data core in the Bank level).
Note: you can't climb pipes with your briefcase, so if you're going for a sniper kill, take your rifle out before you climb. This means you need to make sure no one is looking.

Bumping into guards will separate them and your target, multiple bumps don't make guards hostile.

Always turn electric devices off when you're setting up electrocution kill. Don't break an electric socket and then step in the puddle. Don't activate a taser device while standing in a puddle. Wrong move like this will cost you an ET due to instant death of agent 47.
Do not try to pacify anyone walking the stairs. You will do a judo chop which will compromise you or attract unwanted attention. It's safer to throw something at NPC's head.

Remember the rake in Whittleton Creek? If you're carrying one, do not drop it or it wil break, always place it carefully on the ground.

Some levels have large glass windows, fences or decorative walls with tiny gaps. NPCs can see you or bodies through all of these, so check your surroundings carefully.

Don't walk into a room that has been chloroformed/poisoned without a gas mask disguise. If you don't have this disguise available, turn off the poisoning device if you can.
Note: emetic gas grenade and emetic device are ok to detonate anywhere. 47 is currently unaffected by these (September 2019). Detection timer is longer when you're crouch walking instead of crouch running. Waiter or chef disguise allows you to poison food/drinks without being compromised. Turn distractions on and off quickly to avoid distracting more than one person. You can distract NPCs and make them go where you want them by simply standing in a trespassing zone. Fill their detection gauge just enough for them to get interested but not enough to catch you. This has a far greater range than a simple coin. Propane flasks and fire extinguishers no longer unlock doors.
Most of the time you can use a lockpick (or an electronic key hacker, depending on level security). If for some reason you don't want to do that a breaching charge or an AR/Shotgun can unlock most doors (sliding doors can't be unlocked with a firearm).
You can also use a concussion duck, silenced sniper rifles and assault rifles. If you want to use concealable and silenced weapons, DAK X2 smg and ICA 19 FA Stealth pistol are able to unlock doors after several shots.
A good reason to use breaching charges instead of a lockpick or key hacker is when your level contains locked doors of both types. This way you only use one loadout slot instead of two. Not to mention explosive properties of a breaching charge which can come in handy.
Breaching charge is illegal, so don't let anyone see you use it. Charge explosion itself is quiet enough that no one should notice it but do not stand too close to it because it CAN kill 47. Important: Doors unlocked with explosions of gunshots cannot be closed again!

Krugermeier bullet impact distraction doesn't compromise your position or SA rating but clears out guards (good for SA/SO in Colorado when you need to get into the house basement). This pistol is very quiet and should be used for breaking cameras, shooting gas canisters, distractions etc. Shooting NPC's leg while they are standing in the water results in an accident kill when they fall down. If you see a target standing in shallow water, take a shot from a safe position and you will get an easy accident kill. Leg shots (or lucky object throws) can also make NPCs or targets fall off cliffs, buildings (Dawoon Rangan) or piers (Sapienza).
Coin/audio distraction placed in or near a toilet enables hold my hair challenge completion without the need to poison your target.
Tranquilizer gun pacification doesn't break SA when body is found and NPCs drop anything they are carrying (sedative poison breaks SA and it is harder to target a specific NPC).
Emetic gun (unlocked by doing Haven level mastery) can easily separate a guard from your target or make your target throw up in the toilet without the need to poison them (you need to find a good location to shoot because they do go to a trash bin if it's closer). Make sure no one sees you use the gun though.
However, you can turn this into another opportunity. Overflow a sink in the toilet that your NPCs will go to. As soon as an NPC (if any) has investigated the room, set a taser device trap or follow your target while carrying a car battery. When NPC is occupied knock them out and place them in the puddle, activate your taser or throw a car battery. Note: It seems that NPCs simply walking over the puddle while under effect of emetic poison are invincible to this technique so you need to drag them to the puddle for 100% consistent kill.

Using the strategy above, if you need to complete an electrocution challenge for multiple targets, knock them out, drag them to a sink, overflow it and drop a taser/battery in the puddle. Surprisingly, no one will question that a pile of bodies and an electronic device means accident (provided of course you knock everyone out silently and no one finds that pile of bodies before you complete the kill). In these screenshots I went for simultaneous 5 target electrocution to complete Haven challenges without regard for SA rating.
A bullet impact near the ceiling can sometimes make NPCs go investigate the floor above. You can open locked doors by distracting someone (throw a coin at the base of the door, if it's close enough the door will open when NPC tries to pick it up, even keycard doors will open). Doors will close soon after NPC leaves, so hurry up and get in the room.
EMP device can hack laptops (Sapienza virus control), open doors that require keycards and erase evidence. You can place it or throw it but you need to aim carefully because EMP radius is not huge.

Shoot fuse boxes to initiate distraction from a distance. 47 doesn't get sick when he's close to an emetic grenade but he gets knocked out by his own concussion devices and fails the mission. This is especially dangerous when you're on an elusive target mission. Tested September 2019.
Note: also keep your distance when shooting chandeliers.

You can put an emetic gas device / flash robot in a briefcase and successfully activate them without being compromised.
You can stash a taser device in a briefcase, overflow a sink, drop your briefcase in the puddle you just made and electrocution will still happen even though your taser never physically touches the water.
Concussion duck can be picked up by an npc and that npc becomes a walking pacification machine. You can knock out multiple people at the same time. You won't lose SA if no one is looking directly at your carrier. Same thing happens when they pick up and stash a flashing device in their pocket but you only have 3 seconds to knock everyone out or run through the area (beats throwing coins trying to distract everyone whenever you need to go fast).

Edit: I've attempted a lethal poison injection strategy on a target that has been flashbanged, however, injection prompt doesn't appear. If you knock someone out NPCs will know it was you after they recover from confusion. Therefore, this strategy can not be used for stealth kills, only for clearing a path through heavily populated areas. As long as you don't kill or wound anyone you won't be compromised. Another benefit is target lockdown initiation, you will break your targets normal loop and make them go to a "safe" area where you preferably already set a trap.

Lockdown alarm overrides trespassing detection as long as NPCs are spooked (doesn't affect guards). You can explode a duck and run through trespassing zones full of non-guard NPCs.
You can also make NPCs panic by shooting your gun several times. They will notice bullet impacts and will get scared. As long as you put away your gun you will seem like another panicking person running for safety and they won't notice you trespass. In the picture NPC is looking directly at 47 but is too busy being scared and running towards the closest guard (which also mean you should be getting out of there because guards will compromise you).

Shoot guards from afar (avoid accidental headshot kills) and they will run to the position of your shot leaving their posts without you losing SA rating. When they run to initial shot location they won't know you made that shot (unless you are still at the crime location) so you can run past them without worrying about being compromised.

Throw a duck/phone/coin, pick it up and throw it again. NPC will go to the location of your last throw. This way you can lead NPCs far away from their original location. As mentioned above, NPCs go to the last known location of a thrown object and not where it first landed. So if you want an NPC in a specific location (such as a corner where you can kill them and avoid the "Body Found" issue) throw something that is unlikely to bounce, like a toy tank or a brick. Fire extinguishers knock out NPCs upon exploding without breaking your SA unless someone sees you shoot. For example, you can easily separate Ken Morgan and his bodyguard this way (in addition to various other methods, such as tranquilizer gun).

Some kills require you impale target on a pen or other sharp object. Usually targets have a story that you can follow which will make them lean over it or stand close to it. If for some reason you can't complete the challenge (broken AI, contract that requires you kill an alternative target) you can knock out your target, drag them towards the object, lure a guard to wake them up, then eliminate/pacify the guard and quickly press X to do a context kill. You won't get an SA rating but the challenge will be completed. Save point recommended to try multiple times (unless it is a contract, then you have to do it in one go).