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!)
But mainly get out and surf as much as you can, that's the best way to keep fit for it in my opinion.