I'll tell you what we did with our van and it worked. We had our financing in order prior to going in, like you. Did some research online (used edmunds.com) and found out the true market value of the vehicle based on our area and the condition and amenities (that's not the right word, but I can't think of it right now) of the van.
Then we knocked about $1500 off that price, went in on the last day of the month, 1 hr before close, test drove it, got their offer (which was about $1000 higher than the TMV) and presented our own offer.
They were a hard sell, but we knew our bottom line price was about $800 off that TMV. They moved a little, but not much and I was tired and we had a baby at home so I just said, "look, my bottom line price is X, no need for more haggling and guilt trips. There are a million T&C vans out there and I can go somewhere else and buy one tomorrow. I'm ready to go home now." They said they couldn't meet my price, we got up to leave, were heading towards the door and miraculously, "wait, wait...." They met our bottom line price and the paperwork was done in 20 min and drove our new-to-us van home.
I would highly suggest using edmunds.com for your car research and know YOUR bottom line before you go and where you want to start negotiating. If you don't make any progress, call a spade, a spade and leave. They also had a secrets of a used car salesman article that was a very enlightening read.