Negotiating a severance package after layoff

I was laid off from my job at a New York City ad agency last Thursday.
I received a severance package offer, and believe it can be improved, or should be; but I understand that employers are not legally obligated to offer severance packages in the state of New York. I feel entitled to more for several valid reasons: First, I was solely and directly responsible for retaining a client account that will continue to pay the company a monthly fee until February 2010, even after my departure. A large part of that same fee represents the cost of my employment to the company as the senior consultant on that account. Although I did not have a contract guaranteeing payment of commissions, I was a senior manager there, and worked without a direct supervisor for almost the entire length of my employment (just under 2 years). No one else in the company had contact with these clients or involvement on their business; therefore no one else can take credit for turning around the relationship and retaining the fee.

I believe my former employer should increase my severance package in consideration of this. As it stands I received the standard 2 weeks in accordance with the company's written policy. It is likely I will not be able to find a job on the same level and salary within two weeks in this economy, and these are some of the other reasons I feel the severance package should be improved.

Do you think I could successfully negotiate something better or would an attorney be helpful in a situation like this? It is not clear to me if my situation falls somewhere between the law or just custom, and I would greatly appreciate any advice you may have.