There's not a "form" around for this, and it would be foolish to draft it yourself.
You should have a lawyer draft it to the particular circumstances--at least if you want it to be worth more than the paper it's printed on.
I agree with the other responses: though you will be able to find various "release of liability" forms online, your better option would be to spend a couple of hundred bucks and get some legal advice. Attorneys get paid for a reason. And you'll pay more if you get stuck witn an insufficient document.