Do plenty of stretches. In the morning first thing I stretch my arms, shoulders, back, hips, hamstrings...etc. I do it a few times a day (when I think of it) and always before surfing.
I usen't stretch before, only started the last few months and it makes SUCH a difference. I suffered back problems and it's helped an awful lot -it also really helps your popping up...etc.
Also do 100+ sit ups a day (sets of 25). (Great for your back!)
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But mainly get out and surf as much as you can, that's the best way to keep fit for it in my opinion.