My 2 cents:
-Know the residual value of the car you intend to lease before you walk in. Cars with higher residuals will lease out better. Make sure they know you know that.
-Know the approx annual mileage you'll need. Usually available as 8k, 10k or 12k. Naturally you'll pay more for a higher milage allotment, but it's better than going over where they ding you per mile. If you're not commuting daily you likely don't need 12k/yr but depends how you're using the car.
-Timing is pretty key. Sometimes they have a new model coming soon and they want to clear cars off the lot and will wheel and deal. Try to feel out the dealers to find which makes are running deals.
-Be willing to be flexible. If you lock in on a very specific set of features/color you're limiting your ability to get a good deal. Really want the bang and olufsen sound and black on black? Be willing to give up the cold weather package. There will be prob be trade offs.
-Be willing to walk away. I think that goes for any negotiation.
Godspeed.