Do not try negotiating unless you see the car that you want is available. He might tell you that he can get it in for you, but you do not know if it will be the exact one you want. You do not want to waste your time negotiating for a car that you do not really want.
Be wary of financing if you plan to get another car soon. If you want to get a car without putting any money down, and plan to get a new car in a year or two, you're setting yourself up to carry balances from car to car. You'll end up paying more than the car is worth.
Buying at month's end may be your best bet. All dealerships have monthly quotas. When you come in later in the month, they want your sale to get them to that quota number. You can utilize this fact to your advantage when bartering.