Have your appraisal handy and set up an appointment to protest your taxes. The county will go off what they feel is market. Which could be lower or higher. The appraiser will actually compare your home to others and come to a final figure of what it's probably worth.
Did you just receive this tax bill? You should of received a letter during the first half of last year to let you know of the new assessed value, you then would have had an opportunity to fight it before the actual bills went out. It could be different where you are, but that's the way things work in Houston.
You might not have a choice but to pay they higher assessed tax rate, and then protest/fight the new 2010 rate. If successful, you should then be able to use that same figure for 2009, then get a refund or credit issued.