Thanks for writing! A few notes on GMAT flashcards:
1) Be careful with using flashcards...while it's necessary to be comfortable with the content of the GMAT in order to do well, it's not sufficient. Often we've seen students rely on flashcards as a crutch for more higher-level understanding of GMAT concepts. Accordingly, if you're using flashcards to supplement your strategy study, that's great; but if you're using them as a primary study method, you'll probably struggle when it gets to test day.
2) Because your job on the test is to "know" the content and not just to "remember" it, the best flashcards tend to be those that you create yourself. Flashcards (and hence my points in item 1) are great for recall, but don't tend to lend themselves to deeper knowledge. Because the GMAT tests deeper knowledge (e.g. the ability to reverse-engineer a concept or to find the uncommon answer in Data Sufficiency), simply knowing facts and formulas won't get you much past 500-550. If you create the flashcards yourself, the fact that you have to think first about constructing that knowledge will help immensely.
3) Perhaps most importantly, I'd recommend that you steer clear of looking up concepts that you don't remember, at least until you've tried to prove those concepts yourself. Very few pieces of required knowledge on the GMAT are standalone rules...most of them are items that you could logically derive yourself. If you train yourself to prove a rule to yourself when you need to, you'll: 1) Know that rule much more powerfully because you've had to learn it, and not just read it; and 2) Be better equipped to teach yourself a concept on test day if your memory blanks.
For example, on my GMAT I remember blanking on the rule for x^y * x^z. Was I supposed to add y+z or multiply y*z? Quickly I realized that even an educated guess was likely to hinder my score - what if I see the same thing tested on a subsequent question? - but also that I could prove the rule if I needed to. Algebra is just mathematical logic! I took x^2 * x^3 and realized that it was xx * xxx, or 5 x's multiplied together for x^5. Therefore, I should add, and not multiply. That proof took 10 seconds, but guaranteed that I'd get that question right and feel much more confident the rest of the way. Accordingly, I recommend that, especially in practice, you approach math concepts the same way so that you can truly understand and even replicate them, because short-term recall memory is prone to failure under pressure!