The Washington Post and the article "What to do when an appraisal comes in below the selling price" informs that there are some steps you can take to prevent a low appraisal. First would be to acutally meet with the appraiser in person at the property instead of giving them a key or day code to a lockbox. This way you would be able to give them recent comps and more specific information about the home for sale. For more steps click the link below to read the full article.
Preventing Low Appraisals of Your Downtown San Diego Condo