You should make sure to familiarize yourself with the instructions and the type of prompts that come up. The instructions are always the same pattern: 1, explain the statement, 2, give a counterexample, 3, describe when the statement applies and when it doesn't.
If you follow the instructions, you are basically guaranteed a decent score (4 on each essay, which works out to approximately P). If you don't, even if you write well, you may not do well.
Personally I don't think it's worth practicing a lot to get your writing skills from 4 or 5 to 5 or 6 territory. Just do a couple of practice essays to make sure you're familiar enough with the requirements to get at least a 4.
I just did practice essays when I took a full-length timed test, which was about 3 times, and I got an R. (Doing the full-length thing is important.)