I always just scratch the old sticker off, then rub the glue off with a cloth or my finger... it's usually not difficult. Especially if I'm just replacing the old one with a new one... extra glue is fine as long as the surface is smooth.

I'd use metho or turps (in moderation) if needed, but it hardly ever is.

I always use super glue for fixes and craft glue for stickers. Super glue works... but not always forever. I have had to replace parts with a junker when super glue has failed me in the past.