-Know the spawn point positions to know where they may come from - play a lot of missions.
So you may even shoot blindly onto a spawn point with snipers, if you here them cry something was hit.
-Put fire fields onto spots such as a door where they tend to rush out. They will either take severe damage or, more likely, use a suboptimal firing position and miss.
-Cannon fodder units placed with reaction fire and facing the directions they may come from force aliens to deal with that cannon fodder and attack the fodder instead of plasma blading your colonel.
I don't use gas grenades, as that's about the only really useful thing coming out of the autopsies research tree. Need to really put priorities as research time is so precious on the hard levels.