I think the most important is to get to know the format of Paper 3 first.
Different scientific skills are evaluated in this paper:
- measuring and using numbers
- using space-time relationship
- interpering data
- defining operationally
- controlling variables
- planning an experiment(problem stat/ement/aim of experiment, hypothesis/variables, materials & apparatus, procedure and tabulation of data)
So, the best strategy I would suggest is to go through the above list and identify which one you are still weak in. Go through the past year questions or trial question from different state and try to study why an answer for a particular skill is considered as excellent answer(Score 3).
All the best! All the SPM 2009 Candidates.