Having survived the Midlander Super Grimpeur 300k a few weeks ago, my advice would be:
- Practice, practice, practice. Believe it or not, they do get easier the more frequently you ride them. I consciously add hills to practically every training ride I do. Your body's lactate threshold increases too, which means that you can delay the point at which your legs begin to experience the burning sensation when climbing.
- Contrary to the advice further up, I think getting out the saddle can be a good thing. Not necessarily climbing the entire hill out the saddle, but certainly parts of it. Having ridden with other strong climbers, riding out the saddle is not unusual. Riding out the saddle uses different muscles, and so allows your normal climbing muscles to rest.
- Learn to breathe deeply when climbing. Fill those lungs with oxygen.
- Use your gears. I have seen far too many cyclists trying to ride up hills on their big gears. This is inefficient and wastes energy. There is nothing macho about riding up a hill in your big ring.
- Watch how others ride hills. I have learned a lot by watching other riders, especially those who are good at hills.