i guess you shoud first start with a diagnostic test, tells you how good/ok/bad you are at it. once you have a perspective, depending on the score go for a good prep course. in between it wouldnt hurt to give lots of practice test. I guess you'll be able to find few free practice tests and some very good paid tests online. keep trying all of those to get more practices.
the more you practice, i guess the better you'd get. Here are some references you can use:www.gmatlive.com