There are several considerations when you decide whether to lease or buy. If you can write off the car as a business expense then leasing may be a better option. Most folks though don't negotiate properly, IMO. Instead of negotiating based on monthly payment, I would....
1. Negotiate price of the car (as if you were going to buy it)
2. Negotiate the money factor (near zero is best - the money factor I've seen are from SAAB, I think they're around 0.37 or something rediculous like that)
3. Negotiate the miles allowance
4. Negotiate the residual (lower the better - more options for you in the end or in between in case you decide to trade it for something else)
I think that's about it
If you decide to buy... I guess I would shop around. Usually the 3rd dealership end up with my business simply because by then I know how low the dealers are willing to go
used . When I start to discuss pricing of the car, I've noticed that the dealer tries to hold me "hostage" by keeping my license or registration for my trade. So before the sales guy leaves for the "manager's office" I get all my documents and keys back. When they take too long, I just leave. I've been told that the room you're sitting in may be "bugged" or more likely the speaker phone is left on the listening mode. I guess you could use this as an advantage.... Dealers also want me to see what they pay for them, for which I just push back the invoice and politely say, "I don't car what you paid for it. I'm only interested in what I can pay for it.". This usually leaves them stunned.
I've bought too many cars and as you can probably tell, I like to negotiate :P Good luck and have fun. Just remember, its your money and you can spend it anywhere you like.