OK, so I can't get my chain tension right because my #*(@&!^ wheel keeps slipping forward in the dropout. I even went to a car fixing place (what are those things called? i have no clue) and had them use their fancy air powered socket wrenches to bolt the wheel on tighter. still, one day later, and the chain's got that evil play to it again.

anyone know how to solve this? i was thinking it would help to take some sandpaper and get rid of the glossy grey paint to give it more grip, but that's all i can think of... it's just like the wheel's not gripping on the dropout.