Okay, this is an old question, but I still want to throw out a comment, because someone else will be asking this question again in a couple of months.
I definitely endorse the idea of getting a trusted REALTOR to put together the comparable sales information. Just be sure he uses sales from the year before. For example, the appraisal for 2009 is supposed to be good on January 1 of 2009. If you contest that appraisal, use comparable sales from 2008. Don't use any 2009 sales. The appraisers/review board will throw them out because they happened after January 1.