As with all VMware exams, ensure the latest blueprint is covered in your study. The blueprint highlights all documentation that needs to be reviewed.
Try to see past the technical facts and look at the potential impact if the settings or functionality is implemented in a certain way.
For example, it is easy to memorize the requirements for vMotion; however, for this exam consider issues such as the impact of guest virtual machines not meeting these vMotion requirements, the cluster becoming imbalanced, and the importance of initial guest placement. By considering the content in this way, the final design should avoid these issues.
Read around the technology; VMware.com has several case studies which make perfect reading. Use these case studies to practice drawing out high level designs.
Review your general datacenter and IT knowledge, including basic raid levels, link speeds, and the difference between redundancy, DR, and backup.
If you haven't got real life experience in design work, the official instructor-led vSphere design workshop [v5.x] is essential.
Recommended Study Resources
VMware has a very active community. A very useful series of videos have been produced by professionalvmware.com, where each blueprint objective is discussed. These videos are free to watch online and are great to use while reviewing the recommended reading material from the blueprint. This material is not a replacement for detailed resources such as text books, white papers, and instructor-led training, but the videos are a fantastic tool for exam preparation.
In additional to the official VMware vSphere design workshop, VMware recommend the E-learning course DRBC DesignDisaster Recovery and Business Continuity Fundamentals. This resource is currently free of charge to register and review.