So adhesive removal is part of my profession as an object conservator. The way you want to approach this is from least invasive methods to most.
A sticker is most likely composed of at least three layers. The aesthetic decal, a carrier layer, and a layer of pressure sensitive adhesive. Over time the pressure sensitive adhesive has separated from the carrier layer and created better adhesion with your painted surface.
Two ways to go about its removal. Mechanical and Chemical. The suggestions people are making about using a hair dryer is what I would attempt first. This is basically vibrating the molecules within the adhesive to raise it above its glass transition temperature (also know as Tg). For adhesives, this is the temperature at which they change from hard to soft and rubbery. Once you've achieved this, you can use like a dull scraper and carefully remove the layer without damaging the underlying paint
If this isn't succeeding. You could look at establishing a solvent that the adhesive you're trying to remove is soluble in, but that the underlying paint layer is not. First establish what solvents the paint is not soluble in. I would start with these choices (Water, Ethanol, White Spirit aka Stoddard solvent, Acetone, Xylene, and Toluene). Once you've established what won't damage the paint, try swabbing those solvents onto the adhesive.
