[xPosTech]

If you're talking about the semi-flexible painted bumper I've had good luck with a heat gun. If you can get a hand in behind the dent you should be able to pop it back out.

Keep the gun about 6 or 8 inches from the dent. Heat up the dent and an inch or two of the surrounding area. Keep sampling the temp and flex or give of the dent. When it's hot to the touch and has a little flexibility give it a good push at the back of the dent. It should spring back out and be almost unnoticeable or not at all.

Use good judgment applying the heat. If you bubble the paint with too much heat it will be an expensive fix. Keep the gun moving. If you stop it moving it will bubble.

Do a little research. It's not that hard. When it pops back out you will be very proud of yourself. A body shop will charge you hundreds to do that.

Edit: Yeah just noticed the piston. I'm talking about the dented cover.

Good luck.
Ted