there was an old trick I learnt a few years ago. Drill a small hole in the middle of the dent. Screw in a self tapping screw then get a set of mole grips or plier and pullthe dent. It may be enough to pull most of the dent out and give a better surfface to fill.
http://www.ehow.com/how_4499278_fix-car-dents.html