In general a score of 8 will keep the multiple choice score (out of 800) the same; a score above 8 will raise it; and a score below 8 will lower it. The lower the multiple-choice score, however, the lower the chance that writing a strong essay will significantly improve. A multiple choice in the low to mid-500s often means that the overall score can't crack 600, even with an outstanding essay.
7) Longer is better
Whatever you write about, try to write a lot. One-and-a-half pages is good; two pages is better. There is a correlation between the length of an essay and the score it receives, and the more you write, the higher your score is likely to be.
9) Make sure your body paragraphs are about the same length
Having a very long paragraph next to a very short one creates a sense of imbalance. It's also strange to have a very detailed example next to a very vague one. Try to keep each one to about seven sentences.