Studying for the CCDE is no simple task. It requires a different state of mind than the CCIE and the materials are all over the place. Awesome people like Elaine Lopes, Russ White and Orhan Ergun are working hard to improve the visibility and availability of resources - but that doesn't change the fact that there's a lot to learn. My interest in design and architecture certainly pushed me this way and having a long-term goal to work towards certainly helps a lot.
This post deals only with resources for the Written part of the exam and, while the Practical is not such a huge leap as in the case of the CCIE, it certainly needs a separate post to which I will get to in due time.
Update 2016: blueprint changes to version 2.1 - added evolving technologies section and fixed some links.
Update 201902: updated study matrix links and the evolving technologies book by Nick.
The exam focused part
Then get the spreadsheet and watch the video introducing it - what you have there is an itemized blueprint with reading/watching materials courtesy of the people listed above, amazing!
Watch the CCDE Preparation Methodology Webinar videos, they do a good job at introducing the certification and the two exams - depending on your familiarity with the cert you might want to speed watch them.
Now take that awesome blueprint spreadsheet and work through it, emphasizing your weak areas and refreshing your comfortable areas.
There's also a streamlined book list that you can use to do selective reading on the topics that need attention.
The new stuff that has been added to the exam in this minor update (2.1) is the Evolving Technologies Section. The study resources page is a bit overloaded but there's a lot of interesting stuff there that any neteng should know as general knowledge (if they want to keep up with the industry). Bare minimum, read the excellent document put together by Nick Russo - he adds new content to it all the time so fetch the latest version!
The general knowledge part
All of the above is well and good, but this is not your typical exam where you can focus on a specific set of topics, memorize commands and configuration/troubleshooting patterns and then pass (looking at you, CCIE). And even if it were so, then you'd end up a terrible designer.
Therefore the main focus (at least for me) is to keep an eye out for things related to design which come out of the $dayjob$ and keeping oneself up to date with the industry. Read blogs, news, share your experiences, use the networks you have access to as examples for practical scenarios - does it work as expected, what are the weak points in a failure scenario (and look at its history, there's bound to be a couple meltdowns in there), does it scale with increasing requirements etc.
On this website you'll find articles such as My Big Box didn't save my network!, Keep it simple (1), Resist that temptation and Google's Software-defined WAN and DC, but also more hidden advice related to design in most of my other posts (such as the Leaky Redistribution series).
Other blogs I follow that are worth reading: Danieldib has some great design notes on lostintransit.se, Ivan's ipspace.net needs no introduction and Russ White's writings are epic on both ntwrk.guru and packetpushers.net.
I do realize many people don't have time to read everything on the Internet (I try and fail miserably every single day), so the posts which I think are most relevant for a specific goal will be listed under the Guides section.
Remember, it's the stuff you learn and how it changes the quality of your work that matters, not the number. Although having a bit of extra recognition is pretty cool too :)
And, as always, thanks for reading.