Hide among the dead in Multiplayer
Find some dead bodies, then run up to them and make sure no one else is around besides your teammates. Get down into laying position and crawl into what appears to be the body of the dead person, so it looks like there are fragments of the body around your HUD. Lower your gun while having it high enough to still see what is coming. This trick will make it harder for people to see you unless they are paying close attention to the ground. The flaws to this trick is that people can sneak up from behind and see you since you can only see in one direction without moving. After a few kills, move to another location because other players will pick up on your trick and grenade or rush you.