This trick will cause an Alien to follow you a short distance, and usually
turn its back and appear to watch for enemies. Enter a level that has Aliens.
It can be any level, even a non-Alien one, as long as you use a mpmorph code
to turn into an Alien. When you get to a point where there us not a combat 
situation and there is an Alien around, get near it. Use your Claw attack 
on the ground. Try not to hurt the Alien, then quickly run or walk backwards
to make sure the Alien is following you. This strategy is not foolproof. 
Note: Some Aliens, especially in the Alien Campaign levels, seem to have a
limited "territory." Also, sometimes in the Marine levels an Alien will 
follow you and end up standing in the steam and kill itself. Also, an Alien
on the ground will not follow you onto the ceiling; it will stand by the 
wall.