Severance offer, what should I ask for? - what worked for you?

I was offered 11 weeks of severance after 11 years as a data-entry clerk, and I have a few days to sign. I do not usually post, so bear with me. 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.