L.S.
I just recently went through this (as did many in my neighborhood). A local realtor walked us through what we needed to do. It was actually really easy.
You'll first need some comps -- you can usually get these from a realtor in your area -- of some similarly priced homes. Look for some low end ones because the higher end ones will not help you. You need to look at the price the house actually sold for and the actual price per square foot. Also, if the seller paid anything in the closing (basically the seller loans the buyer a down payment and this is factored into the final sales price) you need to back this out of the final sales price. Also factor out the 6% realtor fees.
Look at your proposed tax statement and figure out what they are assessing your price per square foot to be. Now look at the comps and see what homes are actually going for and note the difference. I argued (successfully as did many others in my neighborhood) that the sales price on the comps includes the 6% realtor commission plus the $$ the people spend to get the house ready to go to market (often this is listed as the various upgrades in the descriptive section of the comps) plus, in some cases, the seller paid amount.
The big key is that these houses (the comps) are market ready and your house is not. I argued that to bring our house to market ready status would cost 10s of thousands to do. Think about it... you'd have to paint the interior and exterior, need new carpet, new counters (in our case), landscaping, etc., etc. I then brought in large (printed on my computer) pictures of all the things we'd need to fix/upgrade. I had pictures of scratched bath tubs, crummy looking sink fixtures, carpet stains, exterior siding that needs replacing, our ancient/original to house AC unit, etc. I basically came prepared with a ton of pictures to show them. I also mentioned that we had had termites that year. The assessor ended up knocking us down about 10% which I thought was great.
Good luck!