Re-do passages you've already done before. Try to figure out what types of questions you usually miss or find most challenging. For example, do you miss main point/purpose questions, or inference questions, or questions involving quotes etc. So, instead of just practicing tons of RC passages (quantity), in this case you would be analyzing and optimizing your process (quality).
Rank the question types you miss often. Re-do passages to understand why you missed those questions, and read specific strategies on how to address specific question types. For example, are you reading too fast/slow, Not reading for structure, mis-reading, having difficulty with mostly science/humanities passages, missing the point etc...
I've also heard that some GMATers use RC passages from LSAT prep. But note, LSAT RCs tend to be longer, badly written (or wordy), and the questions on average are arguably more difficult. However, I do think they could be useful for prep. If you decide to use LSAT RCs: the 1st and 2nd passages are usually easier.
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