Its all about map knowledge and tactical movement.
An aware player will enter a room looking in likely camping spots or strafe around corners expecting someone to be there.
You were probably being a bit too obvious. Did you have camper tools up like Claymores, Betties, or Shock Charges? Like a big neon sign saying "Camper Over Here!"
Some bullshit happens on this game when your being wary guarding a door, people seem to jitter/lagg from the start of the door to halfway in the room. When you watch the killcam the just walk in and shoot you without your character reacting.
Knocked me off a few swarms but i guess it evens out, ive done it to people before, you really know when you get a bullshit kill on this game
Keep in mind the camera issues in this game. There are several posts showing how one person is able to see another on corners, before the other.
You want to camp, adjust for it.
iIve learned when "camping" to keep moving. check your windows and doors often to keep in step with the lag. As stated, the guy moving will have the upper hand if your sitting still. Plus if you're setting up come kill me signs, keep an eye on them and listen for them to blow up. I've caught quite a few players popping my equipment and attempting to run me down.
If you are going to lie in wait, try to be somewhere they wouldn't expect. Dont go to the obvious corner for example, thats the first place people check. Another place people never expect you to be is watching your Sentry Gun's back. Sentry Guns dont just make for amazing bait, they make a damn good set of blinkers too haha. Sure, a smart enemy will just EMP it, but in all honesty, smart players aren't in the majority on CoD now are they