OK maybe you guys are really going to laugh at this one - maybe this is the reason I only think about the next rep now...
I used to work out at a health club in a condo (very nice) where you could actually see the pool through the glass. For some reason, the FIRST rep of a heavy set of bench press has always been tougher for me. So I would see this hot looking girl walking by in a bathing suit and I would pretend I was trying to lift her plus two of her friends. Sorry, I know it's stupid, but it worked.