First book the best rate you can find, cancellable res.
Then look for codes everywhere and anywhere. When you find a code, check the rate. If it's less than what you have booked, book the new one and cancel the previous. You can search for codes, check on Mousesavers, visa.com, BJs or Costco if you're a member, your union if you or spouse is a member of one, etc. Also check the transportation board here. Codes are usually only good for a month or two out.
Keep doing this until you get a week or two out from your trip, and then try bidding Priceline for less than your cancellable reservation. If you get it, great. If not, you can try again several times before you leave. You always have your cancellable reservation as a back up.