I have had some success getting county assessor to lower valuations in the past. Last year the guy even emailed me back and forth a few times and drove by the house before lowering the value for me. Love it. I talked him down another $100,000 over his initial assessment and can't wait for this year after reviewing the comparables today.
In California the way it works is the value for the tax year is as of January 1 and comparable sales need to be within .5 miles of the subject property, closed between Oct 1 and Dec 31, and be as close as possible in size, age, etc. I usually go on Zillow in January and make a list. I also capture photos of the comparables from real estate web sites. The county assesses values and sends out notices in July. From July to Nov or so one can mail in an appeal. I send a letter explaining why my house is so horrible (original kitchens, baths, etc) and an analysis and photos of the comparables (fancy new interiors, new roofs, etc). The first year I did this I used some foreclosures as comparables and didn't get much traction as they are not considered representative of fair market value. Last year I was especially proud as home prices have risen substantially in my zip code but I was able to talk the guy down.
Get after it!