KY jelly or similar product. Avoid any oil based product. I shouldn't hurt much, if it does then it's probably too much too soon. Try using a small anal toy until there is no pain. You should use too much lube an work your way down.
Well...maybe it was just your "lover" that was a dumb dumb and tried to go in too fast at the wrong time, before you even had the time to relax?! Even with plain ol saliva, if the guy knows how to prepare you, there is usually no pain involved..even if he is really big.