Just try to give yourself enough time to double check your work - easier said than done, though, right? I've generally found with my students that they're most vulnerable to careless mistakes on the medium difficulty questions. They're not so easy that you really can't make a stupid mistake, but easy enough that you may come to a quick, but wrong answer. You're devoting lots more time to the hard ones, so you're less likely to be careless there. So if you're in the middle/late part of the section and find an answer very quickly, just be sure to double check those! Hope that helps a little, and good luck!