SAT Writing Advice?!

Hi! I have a question about the SAT writing section. I am going to take the SAT in January and all that, but my main worry is the writing section. I am extremely streaky when it comes to the 25 minute, 35 questions section. On “good” days, I miss around three questions out of the 35, but on “bad” days I miss around eight. I have the Erica Meltzer books, and they are very helpful. So if you could please list any advice on how to improve my score I would highly appreciate it! (BTW, I am mostly missing idiomatic error questions and the “gerund vs infinitive” questions. Yes, I have gone back and re-read those sections, but I cannot seem to retain them on the test!) Sorry for my bad grammar!