If you think the value of your home has gone down, a good way to start is to contact your county assessor. Go to the Web site and look for an assessment form. They will likely want to see comps or an appraisal report that shows the property is worth less than it was when you were last assessed. If you don't have that kind of proof, ask your county assessor if he/she can provide a free consultation with an in-house appraiser if there is one.

It will be different in each State and County.  A good place to start is the County Assessor's Office.  Do remember that the value may still be in line with your neighbors thus, no change in amount assessed.