<p>Barron’s writing workbook is pretty good. It breaks down the types of problems asked so you know what to look for (e.g. misplaced modifier, subject-verb agreement, usage, etc) and provides 4 practice tests.</p>
<p>I took a prep class which improved my score 100+ points. There are certain rules you can look up online that appear on the writing section frequently, such as:
who/whom
neither/nor and either/or
idiomatic expressions</p>