If I have a sniper rifle, I will sometimes place one on the ground and then go prone and lay on it - anyone coming up me will kill us both, but hey the shock and shit in their pants will say more than killing yourself.
Or of course as mentioned if you are in a one entrance only position - like the mosque tower, put the mines at the bottom of the tower.
If you find yourself behind enemy lines put a mine where soliders spawn, or if you shoot someone place mine on your location and then go somewhere else, the precdicatbility of players reasoning will show you how many kills you can get with this method - a player will 9 times out of 10 come back for vengence, smine them again, if they come back a thrid time then frustration will be setting in and their actions will be even more careless... smine them again.