It is my understanding that appraisals are primarily based on the sale of comparable properties in your neighborhood over the prior 3 months. A 3 bedroom, 2.5 bathroom property on .25 acres is going to be generally comparable to all the others in your vicinity unless your home is in total disrepair.

Disclaimer: I am not an appraiser. If I am wrong I invite a professional to correct me.

ispoildogs is right. Home appraisals are based on comparable properties. However, appraisers make adjustments to the comparable sales for significant differences when compared to your home for thing such as age, the condition of the home, and the quality of construction.

As far as mistakes to avoid, although appraisers are supposed to look beyond clutter, I believe it's always a good idea to make sure the home is clean prior to the appraisal inspection.

The most affordable things you can do that can improve your home's value is to cure items of deferred maintenance, paint the interior and exterior, and change outdated or soiled floor coverings. Depending on your local market, landscaping may also improve your home's market value.

Source: I'm an appraiser.