The bad news is that there really isn't a glamorous solution, other than tons of focused practice. Try to develop a better rhythm so that you avoid unforced errors. I usually tell my students to read every question twice before they start writing anything down, just to help ensure that they aren't missing a word or a modifier (i.e. "x is a two-digit integer") that will lead to a dumb mistake. Once they've found an answer, I encourage them to recheck any basic arithmetic or algebra, and then re-read the question again... just to make sure that they didn't miss something silly. Before you click "next", always ask yourself, "how is the GMAT trying to screw me?" Get into the habit of asking this question every single time, and your accuracy should eventually improve.
It sounds like all of this takes a ton of time, but it doesn't. Re-reading the question only takes a few extra seconds, and it's time well-spent, considering the damage that a careless error can cause on the GMAT. Ideally, whenever you see an easy question, you should actually slow down a little bit, and watch carefully for traps. And most of us usually have the opposite instinct ("this looks easy, so I'll save some time by doing it quickly!").
More importantly, you really need to hold your feet to the fire on your homework sets. If you do a set of, say, 37 OG problems, look through it carefully for the silly errors. Again, most of us have the opposite instinct: we're inclined to focus on the toughest questions, and we ignore the missed questions that we understood. Ask yourself what you misread or miscalculated, and try to figure out where, exactly, you need to slow down... or what sorts of traps you're falling into. The GMAT punishes you much more severely for missing easy questions than for missing hard ones, so you should be more worried about the careless errors than the questions that were genuinely hard for you.
In all honesty, you could use almost any quant problem set to work on these skills. If you haven't finished everything in the OG and the Quant guide, then use those; otherwise, any of the quant sets available through GMAT Club should help you to become more accurate.
Good luck with everything, and let us know how things go on your next attempt!
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