I would highly recommend purchasing 新HSK汉语水平考试，模拟试题集。 published by 北京语言大学出版社. The book is available for all levels and includes a MP3, each book includes several practice tests that are identical in format to what you will see on the test. I took both 4 and 5 last year and am now preparing for 6 and these texts are fantastic. In my opinion they are the best thing to get you ready for what to expect on the real exam. One tip, make sure you adhere to the time limits given when taking the practice test, this will give you a good idea of where your strengths and weakness are.

http://blcup.com/en/list_1.asp?id=3252

Good luck!