The best advice that I can give you for this exam is to review the objectives carefully. As a Windows Server 2008 exam whose primary focus is application infrastructure, you would probably expect to see exam questions on things like IIS, Remote Desktop, and high availability. However, this exam also contains questions that you might not be expecting unless you take the time to review the exam objectives. A couple examples of potentially unexpected subject matter might include secure streaming media and Digital Rights Management (DRM) or SharePoint Foundation.
Exam 70-643 has a reputation as one of the more difficult Microsoft exams. Although there are technically no prerequisites to this exam, I recommend taking exam 70-640 and Exam 70-642 first. Some of the material that you learn while preparing for those exams will make the material covered on this exam easier to understand.