Best advice I can give is to talk to arnold869 ( http://www.airsoftcanada.com/member.php?u=17074
). His Biohazard games in Petawawa are pretty badass.
I’m not 100% sure how it would work in a Doom scenario, but the fun part of the Biohazard games is that the different human factions have different objectives… sometimes complimentary, oftentimes conflicting. This is before the monsters show up. It helps to build the tension, and give a chance for players to explore the environment and soak in the atmosphere. When the monsters do show up, give them small victories and rewards (“oh, 20 bbs, awesome”) and then yank those rewards away (“shit, shouldn’t have sprayed down that empty corridor”).