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If you're not worried about scarring the surface, a razor blade or putty knife will give you the best results. Otherwise, get your hands on a softer plastic scraper. Work on the outer edge of the sticker until you have enough loose material to grab. Razor blades and other sharp scrapers are best suited for use on glass, smooth, hard metals, and similar materials that aren't easily damaged. If you're in a bind without a lot of options, a credit card or similar object can also do the trick. Pull the flap up and away from the sticking surface, being careful not to force it too much. Hold it in place with one hand and ready your scraping tool with the other. Do your scraping with your dominant hand to maximize your efficiency and precision. Push the flat edge of the scraper against the bottom of the sticker where it meets the sticking surface. This little bit of pressure should be enough to break the hold of the adhesive little by little. Getting impatient or scraping too aggressively may cause the sticker to tear, forcing you to start the process over again and possibly leaving behind a layer of difficult residue. Avoid digging the scraper too hard into the underlying surface. If you're not careful, it could leave behind unsightly scratches. After a few passes with the scraper, readjust your grip on the loose edge so that it's pulled taut. Then, go to work with the scraper some more. Keep at it until you reach the far end. If the adhesive is putting up some resistance, try moistening it with warm water (assuming it's been applied to a waterproof surface). This method is useful for getting rid of slightly older stickers that are too stubborn to peel off by hand, but can't or don't need to be treated with heat, oil, or moisture.
Pry up the corner of the sticker with a thin utensil. Lift the loose corner of the sticker. Run the scraper under the sticker using small movements. Continue pulling and scraping until the sticker comes free.