Here are my recommendations for VCDX candidates based on my experience with the process and what I did to get through. I was successful at the Palo Alto round of VCDX4 defenses in August 2011.
1. Read the blueprint and make sure you totally understand it
2. Read the application and make sure you cover everything and totally understand it
3. Read the vcdx tips document and follow its advice
4. Know your design and all the reasons for your decisions like the back of your hand
5. During your preparations and lead up to the defense refer to items number 1 - 4 regularly
6. Make sure you complete all aspects of the application and submit all required materials the first time around. Don't underestimate the time you might need to prepare. Your design should cover all areas required by the blueprint.
7. In the defense a lot comes down to your soft skills, so do a mock defense with a group of critical colleagues that can act like customers. They don't need to be super technical, but they should ask you to explain everything and why you have made certain choices.
8. Don't ask a panelist or other VCDX for specific help, we can't give it to you and you shouldn't need it. Just follow all of the documented advice that VMware gives you.
9. Don't think you have to know everything about everything, you don't. But remember to say you don't know, and where you'd go to find out. Use all the resources at your disposal, like you would in real life.
10. Finally write down and read regularly a vision of the successful outcome of the application and the defense. Describe in detail all aspects of how the defense went, how you felt, and how you responded to the panelists and the questions / situations of the defense. It's amazing what this can do to your subconscious mind and how powerful it can be. The more real and emotional you make it (and the more you read it) the more likely you are to believe it. I've used this technique many times and in many situations and it has worked every time, but I guess this doesn't necessarily mean it'll work for everyone. (I wouldn't bother trying it for the lottery though)
Brian Rice posted a great FAQ for everyone thinking about VCDX, and for those that were't successful before on the VMware communities today at http://communities.vmware.com/message/1813247.
You will learn a great deal that will make you a better all round IT professional and architect by going through the process. I learned a heap when I did my defense as well. Even if I were unsuccessful the learning experience alone would have made the whole process worthwhile.
If you decide to take on the challenge of going through this process I wish you the best of luck. Even making it through the application stage is a massive achievement. It is by no means easy, but the reward at the end is indeed worth the effort.