Very interesting question. I'll tackle the specific question about taking breaks. I found that after taking about a month break that upon return, I had regressed. I think the key to solve that is to focus A LOT on the fundamentals. Drill those fundamentals into your head so that you know them for the rest of your life, then study the advanced stuff. This will enable you to more easily refresh the topics which you haven't looked at for a while.
If I really hit the MGMAT Math books hard (at least in my opinion), in your opinion what is the next best math book? I feel like my math has gone up slightly (high twenties to an average of low thirties) but nowhere near enough for how much I've focused on it 75% versus about 25% of my time on Verbal. I consistently see success stories on the forums but they always seem to be for people who bump up their verbal whereas I'm fine with my verbal score (low to mid thirties). I think if I could bump up my math score, I could definitely see more of an improvement in my score.
Edit: GMAT practice tests and real tests
March = Practice (Gmat version 1) - 470, 490 (q27, v31). Actual GMAT: 530 (q33, v29).
April = Practice (Gmat version 1) - 520, MGMAT practice of 650.
May = Practice - 620 (gmat version 2 prep), day before test (gmat version 2 prep): q35,v35 so 580. Actual GMAT: Q32, V35.