Spritz the sticker and the surrounding area with a little WD-40 or Goo Gone, letting it soak in for a minute.
Warm up the decal or sticker and the immediate vicinity with your heat gun or hair dryer.
As the sticker warms up you'll feel the surface become more pliable. When it does, start at one corner and slowly and carefully peel up the edge.
As the sticker lifts, apply a little heat to the leading edge. If the sticker starts to melt, ease up the heat. If it tears at some point, don't panic; you can remove the remnants later.
Once the sticker has been removed, wash down any leftover adhesive with WD-40 or Goo Gone, then use a clean rag to wipe the residue away.