THis is a topic that gets talked about a lot. Some of the more hyped methods are dry ice or similar, suction cups but I'm somewhat skeptical of these.
Perhaps the most widely used in the trade, and what is usually referred to as PDR (Paintless dent removal) is the use of various levers, tips, spoon thingys etc. The technician gently massages the dent out by pressing judiciously on the inside of the panel. The trouble is that even the pros make mistakes, and end up lifting an island...