The Silent Walk

As I was playing through the game I noticed at one point that pressing both the W and D key at the same time to walk diagonally gives you the ability to do "The Silent Walk". This works for humans, androids and the Alien that are near you on the floor and can prevent the Alien to come out of the air vents altogether. Just make sure the enemies don't see you.
This can be used to save time when doing a second or third achievement playthrough.
This works most of the time. Use at your own risk and only when you have played the game at least once and know the level layouts. Also note that doing turns with your mouse could produce footsteps that could alert enemies to your presence.

Preview video on Hard difficulty:

Note: This often also works with the S+D keys, but not with the W+A or A+S keys.