Rough up the surface a little bit with some sandpaper, then soak it down good with WD-40 and let it sit for a while. Then gently scrape it off with a putty knife.
Warming up the glue with a heat gun or hair dryer can work too, but now days most bumpers are not made of metal so you run the risk of doing damage to the plastic with the heat gun method.
Many thanks. I have both a 2000 and a 2004 vintage Bush bumper sticker on my car. For the time being I'll leave both on, to torture our numerous local libs. Once I make my pick for 2008, I'll remove them.
For the time being I'm leaning toward "Vote Neo-Con Conspiracy: Perle/Wolfowitz 2008!"