If you go in at a busy time with the sort of deal I describe here, they will actively stall or ignore you in favor of potentially more profitable prospects. Nonetheless, be prepared to wait them out. Bring some reading material. In my case, they had no other customers, but still turned what could have been a 15 minute quick sale into a 3 hour waiting game - with the same result. This will at least reduce the chances of your being stuck with a neophyte who can't correctly write up a hub cap.
Some sources also suggest approaching the fleet sales rep.
In my case, I was rather open. I told them that I had been looking at Chrysler mini-vans for 5 years, and had walked away every year. I had selected this dealer as the place where I would buy - or walk again, and told them so. You may not want to be this up-front, but it does make them realize you are a prepared and serious customer. Before making the bid, itemize the extra charges.