It depends on how old and how weathered they are, but they should come off with a plastic scraper and some Invisible Glass. When I detailed professionally we had a nice heat gun at our disposal, but even a hair-dryer will do. As long as you warm up the glue enough for it to melt a bit, you can pull or scrape them off with ease. A heat gun is more than necessary, and will melt/burn the sticker itself, your paint, or even your bumper (I melted a 911 Turbo this way).
If the decals are on the window, just spray around 'em with Invisible Glass and go at 'em with a razor blade; if they're on the body, do the same thing but with a plastic razor. Heat helps, and just scrape loose the edges of the sticker, then start from one end of it and pull while you scrape it from underneath. The Invisible Glass (or whatever cleaner you have on hand) just helps to lubricate the blade so it doesn't gum up with the glue residue.