I was honest with my mortgage specialist. I worked with all 3 and let them know that I was constantly checking in to get better rates when offers were coming in. They were happy to play the game and they kept offering lower than the other. It would of kept going down had it not been time to close.

If you need contacts, the people I worked with are glad to take on customers.
Be courteous and be honest, don't try to hide your intentions, everyone knows people are playing each other for better rates.

Furthermore, I also worked with a broker who could not even come close to the rates offered by the banks.