The link from arora is nice, but it's too much work for me.
If you're lazy like me, you have photos of pots there. After opening pedals, spray the contact cleaner in spray inside them (not too much, but don't be shy with it). Then fully press the pedals few times (15?20? you can do more), so the cleaner gets everywhere and cleans whole working range of potentiometer. Then wait about half an hour, so it dries out and you can go back to playing, everything should be fine.
I had problems only with accelerator, but I cleaned all the pots when I opened my pedals. Using contact cleaner might be better than cleaning it manually, because it not only cleans the pots, but also oils them.