Remember, though, that bullets actually lag, though it's hitscan (it is, right?). I aim and follow them with my crosshair, but I let go a split second before I would normally kill them. Usually, the bullet impact seems to lag for half a second or more.
Also, like Goldfish mentioned, people tend to hold down fire even while they don't have a sure shot. A rule you should abide by is that if you can't see the alien in a fight or if you don't have an idea of where it is, don't fire.