Stiff 28-135mm IS

I have the 28-135mm IS lens with my 400d and various other bits. The zoom ring is a bit stiff, and the focus ring seems to be a little bit sticky (but a lot less than the zoom ring). There is also a little bit of dust inside the lens. :'(

How can I repair this? Sell the lens and buy another? If there was an easy(ish) way of sorting it out without opening up the lens I'd do it!