I took the DM exams last week. I passed all on the first attempt. Lowest score on any was 85 (only one). (All others were 90 - 100). Here are my thoughts:
Physics - I agree with the above post. You have to study the Encyclopedia of Recreational Diving and test yourself with the Diving Knowledge Workbook. You MUST memorize the formulas and know how to apply them. (I scored 90 on this one).
Physiology - You just have to read the Encyclopedia and memorize it. Make sure to know about DCI. (My score - 95).
Equipment - Encyclopedia and Knowledge Workbook should be all you need to study for this one. Make sure to really know about regulators, parts and how they work. (My score - 100).
Dive Skills and the Environment - This test was my worst. All I can say is make sure to re-read and study your Rescue Manual, Open Water Manual and Advanced Open Water Manual! Concentrate heavily on dive skills! I studied the Encyclopedia way too much for this exam and should have studied the other manuals that I mentioned. (My score - 85).
Decompression Theory and the RDP - Make sure to know how to use the RDP AND the wheel. The wheel is not hard to master but don't wait until the night before the exam! The practice problems in the wheel manual are adequate to study from. Also be sure to know dive theory basics well. (My score - 95).
Divemaster Conducted Programs - Our instructor gave us the IM to use (borrow) for this exam. You MUST use the IM for this exam. (My score - 90). I wish I could tell you why I scored a 90 on this exam but I can't or I'd basically be telling you the questions. So, all I can say is be VERY careful and double check your answers that you find in the IM!!
Assisting with Student Divers - You can pass this one by just studying the Divemaster Manual and knowledge review. (My score - 100).
Supervising Certified Divers - same as Assisting with Student Divers (above). (My score - 90).
In my opinion, to be able to know AND understand the material well enough to pass the exams, you need to have these items: Encyclopedia of Recreational Diving, Diving Knowledge Workbook, Divemaster manual, Go Dive Open Water Manual, Advanced Open Water Manual, Rescue Diver Manual, RDP and instructions, the Wheel and manual that comes with it, and access to the Instructor Manual (for the Divemaster Conducted Programs exam).
Hope this helps.