I had the exact same issue. If you're falling for the same trick over and over as someone else said, then you need to rectify that and do lots of that kind of math problem. If you're making careless mistakes as mentioned above, you need to do a lot more math problems to solidify your skills. In either case you need to do a lot more practice. It doesn't matter if you know how to do the problem; for most people that's not enough. You have to do a lot of practice in addition to figuring out your mistakes.
Any SAT workbook should do, whether it's Princeton Review, Collegeboard, Kaplan, Barron's, whatever. Do one section from any of these books daily, or two if you feel especially ambitious and have the resources. You're going to keep getting 4 or 5 wrong for a while but after a few weeks you'll notice the number start to lower a little. Just do it up until test day. After doing this for three months, I pulled up a score of 660 to 760 - take my word for it, I didn't believe I had to do so much repetition either, but the work paid off.