It is feasible to go for a refinance when you have built up at least 10% equity in your home (For Fannie Mae owned mortgages, the value is 5%). It is also possible for you to choose the option if your equity is less than 5%, but you may have to pay a certain amount of cash in order to make up for the difference in equity.
Get your interest rate and closing costs in writing as soon as you decide on a lender to work with. Get your lender to give you a commitment in advance of all of the costs that will be involved with your loan. Find out if the refinance loan you are getting has a pre-payment penalty as well. Sometimes lenders will leave out important information like this, if they think it might scare you away from refinancing with them.
If you are making payments on a long term loan, say, 30 year mortgage for the past 10 to 20 years, then refinancing to another 30 year loan will not be a good option as it may increase your overall payment.
Carefully review the estimated closing costs. If you decide to lock in the rate, the lender will send you a "good faith estimate" of your closing costs within three days. Go over the numbers carefully, and compare them to the ones that appear on the final settlement statement (the HUD-1) from your previous mortgage.