1st and foremost, make sure you read and understand every line. Every dollar figure you see, make sure you understand and know about it and what it really is, some things can be presented as this or that fee and really translate to dealer profit. So just make sure you know where it is all going.
BMW's tend to have good residual rates (which determines the buyout). The residual is what you pay at lease end if you want to buy the car (thus the term "buyout" or "buy it out"). The higher the residual, the lower your payment (assuming the same finance rate, which for a lease is translated to a "money factor"). So as long as you don't intend to buy it after the lease, you want the best residual possible.
Edmunds has a some good information on how to calculate a lease and what all the components are, check out here:
http://www.edmunds.com/car-leasing/c...e-payment.html
The residual and money factor is set by BMW and vary by model. The dealer doesn't control that, so your biggest concern with the dealer should be getting the biggest discount off of MSRP you can get and make sure you read all the terms and understand any up-front ("due at signing") fees, make sure they didn't sneak anything in there to give you a lower payment without your knowledge. I've never had this happen with a BMW dealer, but some places will sneak a "cap cost reduction" in there that is included in your due at signing fees, out of your pocket. This is just a down payment. So like I said, make sure you understand everything on the sheet.
Oh, and NEVER pay for floor mats!
Good luck.