I would recommend practicing with other books - definitely get the Official Guide if you don't already have it, and Cracking the GMAT
, the one with the CD. Download the Powerprep software from mba.com. With that and the 5 tests from the Cracking book, you'll have 7 tests - definitely do more than 3 or 4.
The Cracking book will prepare you really well for the types of questions and how to quickly get the answers. The OG has a bazillion (or so it seems) practice questions and is a must. If you spend an hour or so every weekday doing practice questions and do a full practice test each weekend, you should see definite improvement.