If your car is not badly dented, you can DIY knock it back.
Here's my experience in fixing a big dent on my car:
http://img164.exs.cx/img164/65/dentsm1qf.jpg
As you can see, the metal has been pushed in BUT no paint chipped off.
http://img109.exs.cx/img109/4857/aftersm2pk.jpg
After: Dent fixed. However, since it was my first time, I pushed on the dent at too many places, making the metal uneven (as you can see from the reflection).
Tools: Rubber mallet, toilet plunger.
Process: From the inside of your trunk, feel for the dent. Use the plunger to suck out the dent from the outside. Apply gentle pressure on the dent and give a sharp pull. Once the dent is more or less "pulled back", use the rubber mallet and hammer GENTLY from the inside. DO NOT APPLY a lot of FORCE as it will make the dent area uneven and cause paint chips. And hammer the dent area uniformly. Don't focus too much on one area.
Happy DIYing!