Can't get tyre on / Tight rim

gatorskins are a pain to get on, but you just need to be persistent. alternatively, you could try a crank brothers speed lever.

These are great, I didn't understand why they got such good praise until I got one. They pull out and rotate - clip one end over the axle and the other over or under the tyre (different edge for on or off), then just push it around the whole rim. Makes it a doddle, or with the few tyres I've used it with anyway.