This is a free website where you can write your AWA and have a computer score. Once you receive your scores you can play around with it, changing it and watching how you can raise you score with simple modifications.
Link is: http://www.findscore.com/write/selecttype.php
This site is not only for GMAT but also for TOEFL, IELTS or GRE.
You will be able to see the criteria that are used to score your essay, aspects as word count, total unique words, lexical density, number of sentences, average words per sentence, hard words, fog index, among others, with an explanation of each section.
I have found it useful hope you too.