Ben Britton in Battling the Sticky G-sharp Key:
The conventional wisdom that gets passed around is to clean the pad with pad paper or a dollar bill, but a bit of liquid soap and water (or just some water in a real pinch) can go a long way. Use a toothbrush if you can to be thorough.
Ben has some helpful tips to help un-sticking the G# key. I was familiar with the Dollar-note (ie.coffee-filter) and the oil one, but soap/water was new to me. I will give that try next time the key acts up. Which is probably tomorrow.