Allison, both Irina and Bruce have provided some excellent suggestions.
Here are some ideas that might help. (I do not know your market, so discuss with your Realtor).
Normally appraisers use "closed sales" as comparables. Those are properties that have closed escrow and have new owners. The theory is that there properties, since the escrow is closed, actually are the most accurate assessment of value.
Properties that have sold generally also have these characteristics:
1. Escrow if normally about 30-60 days.
2. Market time is normally about 60-90, but varies widely.
If the above were true that means that homes that have sold are actually four or five months old.
That means if the market is trending up, the prices at which they sold might be lower than the current value.
If the market is trending down, the prices might be higher than the current value.
Most areas of the US are experiencing a downward trend, but there are area, pockets, that have trends different than the local area.
Your Realtor should have met with the appraiser at the home for the appraisal and provided comparable sales to the appraiser. Some Realtors only use the MLS, which may not have all the data needed.
Calls to make:
1. Talk with the Realtor about the comparables used. Sometimes there are "better comps" that have not yet closed, but are in escrow, that might support your price. Your Realtor will have to contact agents with Pending Sales to find out that information. It's work, but it's worth it.
2. If your Realtor is not helpful, as to speak with the Broker. The Broker generally has more experience and may be able to assist you. Knowledge is key.
I hope this is helpful.