Disclaimer: Please note that the OCG may barely cover enough content to get a pass. Using these additional resources should cover the gaps in your preparation. Also, I'm not a designer. Still, I would advise all network engineers to look at the design stuff (excluding the marketing information).
What it takes to pass CCDA with flying colors (in brief, see further below for details):
(1.) Make your study outline based on the exam blueprint at cisco.com
(2.) CCDA OCG
(3.) ARCH FLG
(4.) Design Zone
In my experience, some material that was not covered within either the CCDA OCG or the CCDA FLG was found explicitly stated within Design Zone or the ARCH FLG.
The first time I took the test, I had not consulted the Design Zone or the ARCH FLG. In reviewing areas that I was not prepared for on the first test, I found the missing information in the Design Zone or within the ARCH FLG. Also, I realized that I didn't know the OCG as well as I should have. After reviewing the ARCH FLG and the Design Zone for fuzzy areas, as well as really scrubbing through the OCG again, I was able to pass on a retake.
You see horror stories on the Cisco Learning Network, where people basically state that you have to "fall in love" with the material in order to pass. I'm not sure if it's love, but I can state that I developed a much greater appreciation of how things "fit together" after I looked over the ARCH and Design Zone material. And that's what you want out of this, right? If so, make sure to include Design Zone and ARCH FLG in your study plans!
With that said, here's how to know when you're ready for the test:
(1.) You know all the end of chapter questions from OCG
(2.) You know the memory tables in the OCG
(3.) You've studied the ARCH FLG
(4.) You've reviewed the Design Zone: data center, campus, remote at a minimum, plus any weak areas
(5.) You can visualize how any blueprint technology impacts the network
(6.) You make references to gathering business requirements and characterizing the network in casual conversation
(7.) You no longer want to puke when hearing how the Cisco acronym-of-the-day will power your self-defending network

(8.) It starts to make sense (minus the marketing stuff ... CCDA is full of marketing stuff)
About other materials (in case someone wants to know):
I did also look at CBT Nuggets, Skillport, INE, Boson, and Pearson.
Disclaimer: I'm giving a PERSONAL opinion. Others are ENCOURAGED to disagree. My opinion is based on comparing these preparation materials to the questions that I was asked on the exam.
Of these, only the Skillport seemed useful, and it repeated content that you would get in the CCDA OCG.
CBT Nuggets is OK, if you want an "outline" of the content, so that you have some sort of framework. It is not enough otherwise!
The INE course, I think it's by the same guy who did the CBT Nuggets course, and seems to be for an older version of the test.
The Pearson bundle was worthless. Its questions were not of an appreciable level of difficulty. It also included the OCG questions, but I already had the OCG. (I somehow tricked myself into thinking they'd have a superior test engine with some good questions ... NOT!)
Boson questions were of a similar level of difficulty to the real test, but they didn't cover the same pool of content that is covered by the exam. My high 90's scores on first-time exposure to the practice tests did not mirror my result when I took the exam the first time. I was disappointed, because Boson usually covers all the nooks and crannies.
Hope this helps.