The only thing you can really negotiate at this point is a "retention bonus." So my advice is to negotiate a bonus for 3-4 months that is as close to your annual salary as possible. I did something similar with my last company, but I didn't hold out for enough compensation. In hindsight, the deal would have folded without me, but I wasn't confident to ask for enough.

In fact, here's a note to founders: if you promise your employees a year's salary in case of acquisition, your people will stay with you. Say you offer that in conjunction with options, you are already a better employer than 99.9% of other founders.