First, find out the percentage for your town. Some towns are at 120 percent evaluation and others under a 100. What I suggest is getting some real estate comps that are recent. Once you have an amount use the percent to gauge how over assessed you are.
Example: comps come to 300k, you are assessed at 340k but your town percent is 110 percent. That means they can justify your assessment at 300k times 110 percent (330k)
You are 10,000 over assessed without question (as long as the comps are accurate)
Then give the town assessor a call and let him know your concern. Let him know you are planning to appeal your assessment but was hoping that he could correct it without going to the county.
Some towns will give you a fair assessment and others will play the cards at the county.
I helped a friend who was at 430k, town was 100 percent, got low ball comps from 340k to 375k.
Negotiated an assessment of 365k in a five minute phone call.
Oh, if you are only 10k or so over evaluated like the scenario I started with, let the town know that you are going to appeal, have some other comps coming in but would be satisfied with a 300k evaluation if they choose to do so without trying the appeal. Many will go for it to save the work fighting you.