[cheshired5]

A microfibre cloth soaked in hot water which has a drop of Fairy Liquid in it.
Squeeze most of the water out of the cloth then go over the buttons repeatedly until they free up.
A gentle scraping with a cocktail stick in between may help if the sticky residue is bad but use it as a last resort.

Don't use any silicone spray, WD40 or indeed any oily aerosols.