Steam.
I work at 3M Trimline. I remove stripes, decals and graphics of all sorts every day.
I use a "Jiffy Steamer". Its designed for the garment industry, but works fantastic. On most vinyls, it will even lift the adhesive leaving you nothing more than a few swipes with windex and paper towel.
Clothes Steamers, Garment Steamers, Fabric Steamers, Travel Steamers
We use the standup 'personal' version. In a pinch, an electric kettle would do the trick.
When you're done, you won't be able to tell there was ever a decal there, as long as it hadn't been on long enough for oxidation to discolour the surrounding paint.
DO NOT USE STEAM ON GLASS UNLESS YOU HAVE A SPARE WINDOW YOU'VE BEEN ITCHING TO INSTALL.
Glass does not dissipate the heat very well, and you can shatter it with too much steam.
Decals on glass are best removed with a razor scraper and plenty of water. (quarter teaspoon of "Downy" fabric softener in a one quart spray bottle makes a good lube for scraping decals from windows)
Keep the window nice and wet. Comes off slick as a whistle, but you may have a bit of glue to remove afterward.
Decal Removal Tip 
