The way I prepared the exam was the following:
I went to the exam description page (http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-515) and examined the "Skills Measured" section. For each "skill" in the list, I
made sure that I was knowledgeable about the subject, or I researched the topic online (mainly on the MSDN library) until I was comfortable enough with it. I then took the exam.
I didn't think that this exam was difficult, but then again I have been developing asp.net applications for years, so I was quite familiar with the topics of the exam.