Some tips: read the exam objectives and the spec. Skip almost all about EJB2.1 and earlier versions. The spec is very theoretical so a very good supplement is the tutorial of Krizsan: http://www.slideshare.net/krizsan/ocp-jbcd-6-study-notes. This was very helpful! Many thanks Ivan.
Further I used EPractize and Enthuware as exam preparation. I've read several questions of users regarding EPractize and Enthuware. Here is my personal opinion about the two. EPractize has a theoretical part that matches the exam objective against the specs. The Basic and Advanced Sessions guides you through the theory. Enthuware on the other hand has better matching questions and excelent explanations why answers are good and better though why answers are wrong. So both EPractize (provides some theory) and Enthuware (better matching questions) are somehow complementary.