About two months before I went for my DM course, I read for about half an hour each day in my work lunch break, then in the evenings, read and studied for about 2 hours... I went back to the open water manual, the rescue manual, worked through the knowledge workbook, read through the enyclopeadia and re-read the knowledge reviews.
In addition, I went swimming two or three times a week, I went running two or three times a week, stayed of the ciggies, stayed off the booze, ate a good healthy diet... I looked and felt really good.
I did make one or two 'silly' mistakes in the exams, but moreso to misreading the questions... or misreading the answers...
When I went for my IDC, two or three months beforehand I went back through the manuals upto DM, I did all of the exams over and over, re-read through the Encyclopedia and all the materials, watched the DVDs...
In my final exams I smashed it... 100%, 92%, 100%, 100%, 100% and standards..90%
I personally don't think it is fault of the Instructor(s)... it has to be down to the individual... on my IDC there were people sitting up til 1.00am or 2.00am studying, they were waking at 5.00am to study more... I kinda felt that I may be doing something wrong as I was happily going out for (quite..) a few beers each night and waking at a comfortable 7.00/7.30am... I spoke with my course director and he said "at this stage, you either know it or you don't.. if you have to start studying now it's gonna be very difficult.." but I really couldn't learn or take in anymore...
Results were clear to see, whilst all but one in my IDC class passed, most of those just scraped in...
If you want the results you have to study, if there's something that you are not understanding then ask your instructor... instructors should be able to explain in a 'non-diving' environment as well as a specific examples as to how/why/when it is relevent in diving.
I am 41 and hadn't had to study for over 20 years, and.. from memory I hated studying at school.
But, if you want something badly enough, then you have to prepare yourself and put the hours in, you can't turn up to do the DM course and start on day one as if you were doing the OW course whilst on your holidays....
I did my DM course in 13 days and that was tough going, though it would have been so much more difficult had I not done the study or swimming practices beforehand. In hindsight, I would have allowed myself more time.
I hated my DM course as I had so much to do and the days were very long... but I LOVED MY IDC....
Good luck......