Be prepared to walk away if you don't get what you want out of it.
Make sure you are indemnified, that the sale is final and that there is no obligation on your part that survives the contract.
Make very sure that you have all the IP that you sell sewn up tight.
Make sure you keep all correspondence, prefer email and such above conversations about pre-contractual stuff.
Put a person that is not emotionally involved in the deal between yourself and the other party during the actual negotiations, unless you are very good at distancing yourself from this.
Never agree on the spot to any deal, always think it over for a night.
Agree that both parties will carry their own costs up to the point of sale.
Get a professional that you can afford in on the deal, someone that is only beholden to you.
Find a person near you that has gone through this process that you can trust and get their feedback on the process as it goes forward.
Do not transfer any assets prior to the actual sale date, even if it looks like everything is 100% a-ok (I know that sounds like an open door, but that sort of mistake is actually fairly common).
Talk to grellas here.