I would suggest that you volunteer to judge as many competitions as you can find, to get live judging experience while paired with a ranked judge. Go to the BJCP competition page and find registered comps that are in driving distance, and contact the organizer or go to the comp web site to sign up as a judge. Definitely practice writing score sheets on your own, but there is nothing like the experience of judging with some different judges.
Brew as many different styles as you can. I feel that I write much better score sheets on styles which I have studied and actually brewed at least once. Short of that, study the style guidelines, and taste as many of the commercial examples as you can.
The styles tested for are pretty diverse...the last exam I took earlier this year included:
3. Best Bitter
4. Foreign Extra Stout
5. Biere de Garde
6. American Barleywine
(Not an IPA in the bunch!)
Also be as familiar as you can get with common flaws and what causes them. You can study the Troubleshooting section of the BJCP Exam Study Guide, but that doesn't substitute for tasting them in a beer.