When you are offered a deal such as "six months, same as cash" divide the total cost by six. Instead of paying 1/6 ea. month, deposit it in the highest yielding savings or money market account you can find. When your six months is up, pay it in full and keep the interest you've earned.
Be sure to pay it off within the six mo (or time period offered.) If you don't, you may have to pay the accumulated interest for the previous 6 months (depending on the deal.)