- The Alien is programmed to seek you out if you hide in one spot for too long. Even if you are totally invisible to its patrol area it will find you eventually. You want to be hiding only long enough to lose the Alien, then move on. IIRC this is made even worse by crouching? Try to just walk everywhere.
- Ditto if you just hang out in the same area for too long without hiding. Explore briskly and only bother sneaking if you have reason to believe there's danger nearby.
- The Alien is always faster than you, once it's running towards you (it's caught you), it's over.
- Save points have to recharge between each use, so don't be overzealous with your saving.
- Unless you are in a very cramped area it's perfectly okay to just sprint past androids.
- In grand horror tradition, Medical is the worst part of the game. Don't be afraid to pop a lot of items while going through it.
- Keep an eye on ceiling vents. If you see drool dripping from one, the Alien's lurking right there and will grab you if you go underneath it.
- The Alien can hear your motion tracker if you're very close to it. So don't press (or even hold) that spacebar when he's in the same room as yours.
- The Alien is smart, and it's always adapting to your pattern.