I'll second SN2ed. Especially with PS and BS, usually the questions you get wrong are due to your weaknesses... so find out what your weaknesses are and brush up on them! Also, see why you got certain questions wrong and how you can prevent that from happening in the future.
Verbal practice tests are a whole nother beast in and of themselves.
It's more difficult, in my opinion, to devise a plan for improvement on verbal tests, but nevertheless, it's important to see why you got verbal questions wrong too. Often, I would find that I got them wrong because I picked the answer that was "too broad" or "too specific". Even general pointers like that can be helpful.