Sometimes a sticker on your window seems like a good idea, but you later change your mind. Other times, your child can be more fond of stickers than you are. Either way, it is easy to remove stickers from a glass surface.

[edit] Steps

Glass is a very durable surface but if there are softer surfaces nearby, you should start by protecting them. Plastic, painted surfaces, wood, or fabric would be examples of surfaces you should start by protecting. One easy way is to use masking tape and paper -- place paper over the other surface, with a small edge leading onto the glass, and seal it to the glass with masking tape.
Lightly apply a solvent to the sticker. Potential solvents include anything from lighter fluid, acetone, nail polish remover, vegetable oil, olive oil or a product called Goo Gone. Soak a paper towel in the solvent, and rub that soaked paper towel over the sticker until some of the solvent soaks into the sticker.
Use a razor blade to remove the sticker. Ideally purchase a holder for the razor blade called a scraper. Otherwise, carefully hold the razor blade and use a single edged razor blade only
Start at a corner of the sticker. Firmly press the blade against the glass, and slide the blade under the sticker. The solvent will have dissolved the glue and broken its bond with the glass, and the sticker should easily scrape away from the glass.
If the sticker resists removal, try applying more solvent then repeat the scraping step.

[edit] Tips

You can also use any type of oil, such as vegetable oil, olive oil, even Avon's Skin so Soft. Just take off the top of the sticker, the part with the writing on it. It is usually coated so the oil has trouble soaking in. Coat the sticker with the oil, let soak. The sticker should just rub off. The oil is more child and pet friendly.
Erase with a pencil eraser.
Car wax also works excellent (not the liquid type). Apply a coat on to the surface, wait 3 minutes, and then remove the sticker. Liquid wax will not work!

[edit] Warnings

Use solvents in a well ventilated area.
Wash hands thoroughly after using the solvent.
Be careful with the razor blades, they are sharp!
Be careful with the solvent, it will often disfigure plastic, fabric or other sensitive surfaces.
Glass scratches, so only use your blade in one direction
The best thing to do is get a blade that is made of stainless steel as it will not scratch the surface.