So for, example, if you missed a subject-verb agreement question on the Writing section because you were thrown off by a prepositional phrase between the subject and the verb, don't just worry about that particular subject and that particular verb. Make sure you know what prepositions are and prepositional phrases are, where they start and where they end, and practice recognizing them. Go get a grammar book -- it doesn't even have to be mine! -- and do a bunch of exercises until you can pick them out automatically. Take a book that has nothing whatsoever to do with the SAT and underline every prepositional phrase that you see. Since subject-verb agreement errors involving prepositional phrases are virtually guaranteed to appear, you stand a much better chance of raising your score if you know unconditionally what a prepositional phrase looks like. And by "unconditionally" I mean that you can recognize one in question 12 of Section 10 after 4.5 hours of test-taking, without even having to think.
If you can identify all the major concepts you're weak on, focus on improving them one at a time , and not take a new full test until you truly feel as though you've mastered something new, you'll be astonished at how long your Blue Book lasts.