I used http://www.carcostcanada.com/
when I picked up my e90 last year.
What you are basically interested in is the dealer's invoice cost and any factory rebates active during the month you are planning to buy a car.
As a rule of thumb, anything on the dealers quote to you is negotiable to a certain degree. Carcost will give you an idea how much dealer is making on any given car and basically gives you more room to play. If you know exactly what kind of car/model you want then go directly to the sales manager bypassing the sales person. Try different dealers, etc. some will be more motivated than others.
If you are planning to lease, they will always try to ask you how much you can afford per month and play with the monthly payment. This is an easy way for them to make money of you. Spend the time and understand this monthly payment is calculated. Understand the variables that affect it (residual value, lease term, rate, etc.). If you are planning to buy a car, consider leasing it first then buying it out later. Depending on the financing rate you can save some money on the interest.
Negotiate admin fees, lock nuts, floor mats, etc. They will try to charge you for anything. Good luck.
As an example, I saved over 11% on my e90 over MSRP.