First of all: SAFETY
if you're up the top of a face, or on a ledge - make sure you're adequately protected - either with your own belayer or with slings to an anchor(s). Sometimes people get disoriented when looking through the viewfinder so make moves you wouldn't normally! If in doubt, don't do it! No picture is worth even the remote risk of a fall. Climbing is a dangerous activity. I don't know your experience level, but just don't risk anything. Always make sure you're clipped in, and be safe, and never ask someone else to do anything that might compromise their (or the person that they're belaying's) safety.
get up high if at all possible - shots of a climber's arse as they climb and you're on the floor really aren't flattering!
try to show the climber during a move - and avoid where possible the 'easy' shots as they sit at the top threading their belay, or while being lowered off. Looks the gnarliest and most interesting
Sometimes even just getting closer to their level (easiest way: if there's hills or bumps or raised bits of ground around the route):
Getting in close with a telephoto is probably generally good, if only because of access etc, however the occasional wiiiide shot can look cool too: (this one's from my gopro!)
If you're climbing with your camera, put it over your head and on one shoulder, and put the camera behind your back. Then put a screwgate carabiner from a gear loop on the side of the shoulder that the strap's on, through the camera strap, for safety and to stop it from swinging. How I prefer it, anyway. Though ideally, especially if you're just going to a relatively low ledge, just have someone reach and pass it to you.
Add some context - just a person on a wall can look a bit boring. Something like this gives a sense of place:
And finally, sometimes you get great pictures on the approach / leaving the crag, or at random times, so don't pack away your camera:
