So in short, I'd say I'd look at the study guide LincolnWasInACent created and if you fully understand the questions and their subject matter, you probably are ready to take the exam, but if not, I'd recommend using some of the sites I mention below to actually learn and understand Salesforce before taking the exam. See below for more info summarizing my experience and opinion when it comes to studying and taking the exam.

I went through the entire study guide at: http://certifiedondemand.com/ It was quite time consuming and while it gave me a good overall feel for using SF, I'm not sure how well it actually helped me to prep for the exam. I also completed a bunch of the trails at: https://developer.salesforce.com/trailhead This was the most enjoyable site that I used to study and felt that it gave me a good feel for the SF interface. Once again, I don't feel that it actually did a good job preparing me for the exam. I also took many of the random practice exams that you can find with a quick google search. These did help prep me for the exam in the sense that the questions were a similar format and by the time I went to take the exam, I already felt like I had taken it numerous times. Keeping the above in mind, with all the time I spent studying, I felt very confident that I could step into a SF role and perform at a high level, but I still wasn't sure I could pass the exam. This is where LincolnWasInACent's study guide came in. While all of the studying I did prior to taking the exam helped me to learn and understand SF, his guide did the most to help me pass the exam. And because I had already done so much studying, I was able to fully learn and comprehend the material in his guide, as opposed to it just memorizing it. I don't feel the SF exam is a good tool to measure one's knowledge of salesforce and their ability to step into a SF role in a corporate environment. I feel multiple choice exams are a poor measure in general, and felt the questions on this exam were pretty random, often worded in a way that was misleading. I feel that someone who could excel in an actual work environment could still fail this exam. At the same time, I'm sure there are folks who can pass the exam and struggle in an actual SF job. This is my TwoCents, hope it helps!