In reply to Lbos:
1 - Staying on the wall (or crag) for 30-45 mins is great (specific) aerobic training - put the i-POD on; place a bottle of water within reach and then climb continuously for that length of time; up, down, sideways, round and round. You can vary the intensity but the important thing is not to get pumped or just take on a light pump. Over the winter last year I did this once a week and it helped to maintain stamina (not endurance).
2 - Doesn't work so good for aerobic endurance in comparison with 1 and doing Easy problems isn't going to help with anaerobic endurance. But, doing hard problems that are "doable but at your limit" on the minute (60 second rest betweeen) is a good for anaerobic endurance and an alternative to circuits.
2a - Circuits pretty good for building anaerobic endurance. Link 3 or 4 problems together to form a circuit that you can just about do. Repeat 5-6 times with 5 mins rest in between.. You'll be surprised how tough it gets.. you should aim to feel sick, pumped and shagged out by the end of the session.
3 - Doing routes 'pyramid' with 5 mins rest between each one is another good anaerobic method.
Anyway, it's all there in the training books...