http://www.theautoleasecalculator.com/" onclick="window.open(this.href);return false;
I used this when I calculated my lease.
Things I wish I did differently:
1) Put as little down as possible! Your down payment is not recoverable if you total the car. I put just under $2500 down and wish I didn't.
2) Get on the same page with your dealer before you walk in to pick up the car. Before you even walk in, you want to settle: a) your down payment, b) their fees, c) what their money factors are for all tiers. Knowing these three ahead of time will put you in a better position to avoid being screwed. Luckily I wasnt, but I had to do some last minute math to make sure everything added up right.
3) Negotiate the price of the car down for each fee they tack on (like doc fees). Also make sure you aren't paying for anything extra that comes with the car (lug nut locks, nitrogen filled tires). Also don't pay for anything extra that you don't want (tinted windows, VIN etched windows, pinstriping, etc...). *Refuse the extended warranty.* They will pitch you this deal towards the end when you are working with the finance guy while they are running your credit. The protection plan is quoted as an increase to your monthly payment. It's not worth it. People fall for it because it gives them peace of mind. We don't need no stinkin' peace of mind. Just math. It's not a good deal.
4) Try to get your NMAC account number before you walk out (might not be possible) and set up direct withdrawal sooner than later. Authentication took me almost a week.
5) Before it arrives- ask them to not put any dealer stickers or badges on the car when they get it, and ask them to remove the world car sticker from the rear window when they are prepping it. Save yourself the pain of having to remove them later.