Assuming that you have done enough of OG Math (atleast twice with an understanding of why you got some wrong and why you got some right), to improve in Math I sugest -
a) do as many re-installs of GMATPREP as possible because each re-install gives you atleast 75% new math problems (for upto 8 re-installs that I have done). You may want to chose to do just Math if you are ok in verbal.
b) While doing the math, flag every question that you "guessed" or could solve but took long time to solve.
c) At the end, take screenshots of each of the questions that you got wrong and the ones which you guessed but got right.
d) Come back to GMAT club math forum
, do a search on "GMATPREP PS" and "GMATPREP DS". This should give you most of the GMATPREP problems and your selected problems may already be there. Go through them to understand the logic of solving them. If your problems are not there, post them here and you can expect some great solutions from folks around.
All the best.