The first thing to do is slightly wet the area of the dent with water and let it dry throughly. You can also carefully place a household iron set to low heat on the dent when wet. Some or all the dent may come out on it's on.
What ever does not come out should be filled with a super lightweight balsa filler and sanded flush with a product such as [link=http://www2.towerhobbies.com/cgi-bin/WTI0091P?&C=RMC&V=GBG]this.[/link]