How to get this dent to stay out?

[fireball 440]

This dent was here on the pass side door when I bought the car. It pulled right out with a toilet plunger. The problem is whenever I wash and wax the car the dent pushes back in and I have to get the plunger back out.
Any ideas on how to keep it pulled out?
I haven't had the inner door panel off yet but I would think any bracing would interfere with window operation.
http://i199.photobucket.com/albums/aa80/fred_gee/grandam.jpg

[fireball 440]

Ttt

[tenspeed]

Take the door panel off and glue something like peg board or foam insulation over the dent area. If there is no room, try some spray on foam.