Get a lawyer. Not someone small that you've been using regularly for the little stuff who's never done this before, but someone that has experience. Be careful about this; there's a lot of dummies that put on a show of competence.
Same thing with a negotiator. Are you in a city that has a lot of startup activity? Reach out to a good COO type who can help you conduct the negotiations. How much they get involved will obviously depend on the amount of money we're talking about. It's just not a good idea to learn about negotiations by doing in circumstances like this.
Be careful who you pick. Money turns heads unfortunately, and a lot of otherwise sensible people become complete pricks.