The way I figure it, if someone flanks you, you're either injured or dead.  If you run out of cover while someone is on overwatch, you're injured or dead.  If you try to shoot someone while being suppressed, you're injured or dead.  If you don't move out of the way of a burning car, grenade, or RPG, you're injured or dead.

Pay attention to your surroundings, be smart, work as a team, cover your flank, and you'll be fine.

Same goes for EXALT/Aliens.

EDIT:  Oh yeah, and if you're behind some full cover (a thick tree, large van, or wall) you're probably not going to get hit.  If you take cover behind some benches, mailboxes, low walls, etc, you best watch out.  If someone is on a mountain, building, or something, they might have a good chance of hitting you.