I tell him I'll be back when he comes to his senses, and then I leave. You can't get anxious and act like you're in a hurry to buy. If an offer comes close to Vince's pre-set limit, he'll stay and talk, but only for awhile. I'll be very civil, but eventually I'll say 'We're not making any progress here. By then they have my phone number, and I wait for them to call.
Car ownership is a long process.
Buying it is just part of that. If you get your brain wrapped up in dealer financing during the price discussions you'll get confused. The dealer has to wait for you to come in, so use that advantage. Shop when you're rested, not rushed, and try not to wait until you're desperate for transportation. Shop toward the end of the month, when sales people are anxious to fill their quotas.