Most of the time, a "sticker" warmed a tad with a hair dryer first, will peel off and take the adhesive with it, rather than leaving the gunk on the surface. Especially on plastics and varnished finishes.
Naturally, one has to proceed carefully with applied heat.
I've had good results with rubbing or denatured alcohol on some adhesive residues. Denatured alcohol is far more effective than rubbing alcohol, which has a high water content.
If that doesn't do it, then lighter fluid (naptha), mineral spirits, or as a last resort - lacquer thinner. Most of the GooGone paint and tar remover products are based on lacquer thinnner.
WD40 dissolves quite a few adhesives.
