New cables, properly lubricated, and lubricate the insides of the shifter. If the cables have got to the point of rusting to the inside of the outer cables (or anythign else), I would strongly advise replacing them. It's cheap and easy, but very effective.
Alternatively, it may be that your mech has siesed up (sp?). Disconnect the cable to the mech, and try to move it by hand. If it moves easilly, it's probably your cables that are sticking. However, if it's hard to move, that may be your problem.
Hope this helped a bit