I'm doing desertification as well. Not the most interesting topic, but there is a lot of info on it. I really wanted to do animal trading but it was way too hard! I'm using the map in New Perspectives.
Nic K - other advice:
-Make sure that you can apply the spatial concepts well, and use them frequently.
-Know the difference between a policy (aim) and strategy (steps taken to achieve that aim) and know details of them including evidence
-Practise drawing accurate, neat maps TO TIME. You don't want to spend 30 minutes working on a map when it should take you 8! (Think 2 minutes per mark). Don't forget to include BOLTSS.
-Learn to describe distribution patterns well (PQE, which is in New Perspectives at the front somewhere, though you've probably already learnt it!)
-Think about factors for...well...pretty much everything.
-Be able to justify and evaluate policies.
-Know lots of evidence, eg. For the MDB you should know about 20 facts (such as how much irrigation is worth to the economy for example)
-Be able to map your local resource AND map it in regards to the surrounding region.
-Go over past exams and self-correct them. Read the examiners' reports.
They're the main things I can think of right now.