One of the things I've found helps people score higher on Quant is going beyond learning formulas, strategies, etc. for each topic to learning the underlying logic of each of the Quant topics.
For example, in Combinatorics, that would mean going beyond learning the combinations and permutations formulas and strategies to learning why those formulas and strategies work.
Similarly, for Overlapping Sets, you'd learn not only the formulas and methods but also how and why they work along with the logic of the relationships between overlaps and how the different components of a system of overlapping sets change as you change the overlaps.
There are multiple advantages to learning Quant this way, and one of them is that it allows you to apply the Quant concepts flexibly to answer out-of-the-box GMAT questions that involve them in new ways.
For example, someone recently told me that he saw on the GMAT an Overlapping Sets question involving five sets. Of course, none of the standard formulas or methods for two- or three-set questions will work for a question involving five sets, but if you get the underlying logic of Overlapping Sets, then you'll still be able to answer such a question.
Another thing that works is using the streaks method to practice. Doing so, you'll learn to see all the key aspects of each Quant question and thus become both more accurate and faster.
How to Ace the GMAT Using the Streaks Method