Refinancing costs around 2-3% of the loans total cost. If you take the mortgage fees and then divide them by how much you save a month if you refinance your mortgage, this will give the total in savings so you know how long you have to wait until you break even.
Consider any penalties you will incur if you pay off the loan early. Add the penalty cost to your closing costs for just refinancing alone then repeat “tip 1” to be sure that you won't lose any money in the long term should you refinance.
Negative equity is the difference between your current house price, and how much you owe on it, if this is less than the money lender intends to offer you, you then can either not refinance or you have to pay the difference yourself. Therefore make sure you refinance at the optimum time.