This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
During a Certification Boot Camp, I have used a practice exam in group study sessions.
We look at each question without any time limit and take turns explaining the why of an answer.
Sometimes there is real debate in the study group. We find there is always a reason why an answer is correct or a cleaver distraction.
The group exercise can be interesting and builds good understanding of both the tested objective and nature of the exam questions.
Also I have found that a "more difficult practice exam" will usually yield "more understanding."