Chances are, you already know about this little trick, but if you don't, here is what you do:

Get in a doorway and press the backs of your hands on each side of the frame as hard as you can. You really have to push really hard for as long as you can. Your arms will start to shake, but you have to keep going. Once you feel like your arms are going to give out, stop pushing and step out of the doorway.

Disclaimer: I am not responsible for anything that happens as a result of following these instructions. The first time I did it, my hands hit the ceiling fan - that hurt.