Use cotton swabs (Q-tips) dipped in isopropyl alcohol while the controller is turned off. Don't turn it back on til all the alcohol has evaporated. If it's the underside of the controller that's sticky (as in the buttons are sticking when you press them down), you're gonna have to open it up and clean it gently with the Q-tips and alcohol. As long as you don't dump the isopropyl alcohol all over the inside, it's not going to ruin the controller and it might actually make the thing work better overall if you happen to swab out any contacts.