I cannot speak for the HVAC depth as I have not taken it, but as far as my experience on the Mechanical - Machine Design...my 'plan' and some observations:
Planning:
-Bought MERM and accompanying practice manual
-Borrowed 2001 sample exam from a friend
-Used Casio FX-115MS I used 7 years ago for the FE.
-I originally wanted to take a review course to stay structured. As with most of us, I was concerned for the areas that I don't practice, or haven't seen since my undergraduate days. Long story short, my employer wouldn't pay for the course and I decided the $650 was better spent elsewhere. I also figured I could take the exam three times before breaking even. So no review course for me.
-I planned to start studying after New Year's for the April 2011 exam. I was hoping to do an hour or so a day and work through the book. In reality, I didn't crack the MERM until the last week in February. Frantic, I decided to prioritize my studies. Since I was taking the MD depth (my specialty), I figured I'd stack the deck in my favor. I focused on the MERM sections that mattered to me..started with statics, then materials, then machine design, vibes, etc. I personally hate Thermo, so I left that till last. Never actually did Thermo, power systems, math, ethics, etc. I 'prioritized' more than I wanted...
-I would read a chapter of the MERM, then do the accompanying problems in the sample book. I did all English units. I skipped some of the 1 hour problems that I felt were out of the question.
-Total study time...hard to say. If I had to guess, I'd estimate about 150 maximum over 9 weeks.
Test:
-Brought MERM, practice problem book, sample exam, Machinery Handbook, two design books, and a manufacturing book. I used all in some fashion, but I'm not sure which ones I'd recommend again...I'd have to think about it. Def MERM...
-I worked through the exam...no skimming per se, but I did skip problems if I felt they were skippable. Got to the end and started over...rinsed and repeated until I was out of time. Yes, I used EVERY minute.
-Forgot to bring Advil. I had a wicked headache in the afternoon that made me miserable.
Fortunately, I passed on my first attempt. With that in mind, I'm happy I didn't take the review course and I'm glad I didn't study more...it would have wasted my time and maybe clouded my mind with too much knowledge. Yes, it is possible to overthink this exam... Prepare as best as you can, but be prepared that very little of what you studied might show up (my case). There was a LOT more thermo/fluids on my exam, but previous exams have gone the other way. All about what you're comfortable with. All in all, I'm not sure my preparation did much for me. If I wanted to feel good during the exam, I'd change my approach the next time and prepare with more thermo/fluids/heat transfer problems...areas outside of my specialty. Apparently I handled them well...but I didn't feel terribly confident during parts of the exam.
My recommendations...figure out your priorities - both with the exam and your life during the next few months. Do what you can and RELAX! Best of luck!!