I'm detailing a friends car this weekend.

Any ideas on removing a bumper sticker?

Depends on how long it's been on there. If it's somewhat new, hairdryer, heat it up, pull gently.

If it's baked on from the sun from being on there a while. Don't use your fingernails too aggressively, put peel what you can. Wet it over and over, try to get off what you can with your hands, but be gentle and careful not to scratch the paint by using fingernails, etc.

Orange oil, Goo-Be-Gone, products like that can help get rid of the excess/leftover sticker parts. But if you use that kind of product make sure you throw on some wax to protect the paint in that area, cuz the paint will be exposed with no finish on it.

Sometimes lubricants can help too. Might be more gentle too...