helps to use the right killstreaks, too. b52 / chop gunner / dogs. b52 wipes everyone out, gunner starts the spawn rape, dogs take out the guys that run inside to hide so they can spawn back outside for your gunner to get. of course, that really only works on the tiny maps, like firing range or nuketown. otherwise you really can't get that many kills.
use the right perks, too. flak jacket / sleight of hand / tac mask. flak jacket = no random clay / tube / nade deaths, or deaths by those dumbass rc cars, tac mask pro = no stuns or flashes. essentially, you're a fucking tank and can't be stopped. then just don't stand in the open where people can hit you from many different directions, and dont get flanked. do the flanking. move a lot.