Should I negotiate my severance?

I was offered 15 weeks of severance after 15 years as a store manager, and I have a few days to sign. I worked as a store manager before this. It includes health coverage for a couple of months. Did anyone negotiate and succeed?

Ask for the offer in writing and do not sign on the spot. A day or two to think is normal.

Thanks for writing this out. I went through something close to it 5 months ago, and it got easier once I kept a simple log of every call and form.