What to do about a button on a video game controller that ''sticks''?

A PlayStation 3 controller, to be precise. The X button doesn’t work properly. It doesn’t have its bounce anymore when you press it, it takes a while to sink in, and it comes back up slowly. This makes playing games pretty hard, especially with action oriented games where something like jumping can be intensified or repressed, depending on how hard/lightly you press, how long you keep it pressed in and whatnot. The X button on the controller is a main button for many games, so this is sucking.
What causes this, what could it be, and how to fix it? Can it be fixed? I clean the controller regularly, but not its insides…or am I stuck buying a new one? It’s about a year old. Help?