I tape them to the inside of my rear window.
I NEVER put bumper stickers on my bumpers. And If I have to have a window or windshield decal, I carefully put clear plastic on all but the outer 1/8", then tape it onto the inside with 1/4" width tape.
My cars always look like they just came off the showroom floor as long as I own them. Even my previous beloved car (lost to a rear-end collission) after 120K miles looked like it was brand new.
my pickup still sports my pro-Bush NRA sticker... from 2000.
'course, a beat-up old half-ton pickup is allowed to have stuff on the bumpers :)
I bought some magnetic backed paper, cut out a piece the size of my BUSH bumper sticker, stuck the sticker on it and put it on my car.