People think of camping as a cheap strategy to win and get kills, but camping is an effective strategy.  Ambushing common places and hiding at points where no one will expect you be there catches your enemy off guard.   Camping is best for the team that is defending.  On bomb maps, the terrorists are doing the rushing so Counter-Terrorists can guard entrances to bombsites or places where terrorists will pass through.  Likewise on hostage rescue maps, the terrorists can camp common routes to where the hostage are. 

When camping, find spots to camp where no one will expect someone to be.  For example, a passageway with boxes at the side.  You would normally just move along the path and then, you find yourself hit and you are dead.  Spots like behind boxes or walls while guarding somewhere which people move with their backs facing you.  Once you found this spot, you can kill alot of people if they don't notice you.  But remember, do not camp the same place twice.  Try also to make the surroundings to your advantage.  For example, camping in dark places gives you cover.  But shooting will give yourself away so i recommend you silence that gun.

One really good strategy that involves a few people but can take out alot of enemies require one or two campers and a few others.