the belt will work much better if its sinched up real tight. I always sinch it up as tight as I can, sit upright, and try not to move around too much to loosen it. I do this just so it holds me in my seat better, but I suppose its a pretty good safety precaution too.

Something you might try is to move your seat back one notch, tighten up the belt as much as you can, then slide the seat back forward. That'll hold you in good'n tight