The best use for the extra cash is to pay off any higher-rate loans you may have. Let's say that you are carrying a $15,000 car loan at 10% and making minimum payments on a $10,000 credit-card balance at 17%. Your monthly payments on those debts would total $680. Then assume you refinanced your mortgage, taking out an additional $25,000 to pay off your car and credit-card loans. Result: At 7.5%, your additional monthly mortgage payment would total only $175, so you would come out $505 ahead ($680-$175=$505).

Of course, all the extra cash needn't go for paying off debts. When the Jensens swapped their ARM for a fixed-rate last December, they also increased their mortgage load by $34,000, from $106,000 to $140,000. They used $3,000 of the proceeds to pay their refinancing costs and another $17,000 to pay off a 10% home-equity loan, which had been costing them $250 a month. Then they spent the remaining $14,000 to build a garage for Steve's antique-car collection -- and they did all this for just another $19 a month.