For Step I, I found success with combining studying for M2 courses with preparation for the boards. Due to the amount of information that you are responsible for on Step I, becoming as efficient as possible in studying will definitely help. I did this by buying "First Aid for Step I" early in M2 year. As topics were covered in M2 classes I would also look up the relevant pages in First Aid. This allows you get through most of the book by the time M2 year is over. Also, I got the 6 month subscription to Kaplan's Q bank and would do the questions that were relevant to my M2 exams as I would study for them. This lets you get exposure to boards type questions while also preparing for your exams. In addition to those strategies, I also went through the Board Simulator Series (BSS) and NMS question books which I began reviewing a few weeks before spring break. Between BSS, NMS and Q-bank, there are a lot of questions to go through. However, add up the total number of questions and figure out what pace you need to keep to get through it all (for me this worked out to 50-75 questions per day, everyday).
When you are within a month of exam day, I recommend taking the Step I practice tests that are found on the NBME site. The good thing about them is that they both predicted my score within 4 points, however, the bad thing is that they each cost $45 and there is no answer key or explanations for them. So, use them to see where you stand in terms of your preparation and for building test taking stamina and not so much for learning.
After that, the rest is a matter of sticking to your plan. Good luck, and remember to try and save time to do things you enjoy as well, avoiding burnout is huge!
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Alpha Omega Alpha
http://www2.uic.edu/stud_orgs/hon/aoa/