Tape can be the easiest way, if your tape and the the residue are of similar composition. Also, it usually only works if the residue is fairly fresh.
Often, it seems to me that old keyboards have old residue on them, and that is the hardest to remove.
A chemist once told me that it is easy to get an adhesive to stick, but it is much harder to get it to stick and release. And, over time, some turn to dust, while others become ferocious and etch themselves into the surface.