The requirements vary by jurisdiction, but the overall tone is the same.
You need to have the nature of the agreement fully spelled out. A verbal contract would do, but there's no easy way to prove to a third party that a verbal contract exists, so you need to get it in writing. For the same reason, you need to have sufficient information to be able to identify the contracting part(ies)—identifying yourself is easy, of course, but identifying the subject is somewhat less easy, and any third party is going to want a little more assurance that they have permission to run the photo than a name. It's a CYA thing; a name and a deed (a signature or other mark, something that the subject actually does) is all that's legally required in most places, but it may not be good enough for the people who want to buy a license to use the photo.
Where it can get tricky is in the consideration area. You need to get some sort of regionally-specific advice as to what constitutes valuable or good and valuable consideration in your jurisdiction. In Canada, for instance, until relatively recently the picture itself (or a print of the picture) would not have constituted good and valuable consideration for a model release—there had to be a payment, and the amount of the payment that would be considered sufficient varied by the apparent intent at the time of taking. (A nominal amount, like a dollar or even a penny, might do on the street, but for anything done in-studio, you'd need to crank it up to a hundred bucks to make the contract enforceable. To do TFP, you'd need to spell out the value of the prints in the contract to make sure it met the hundred dollar requirement. But that's now just historically interesting.)
Get local advice. If you have a professional photographers' association locally, find out what they're using. If not, you may have to enlist the aid of a lawyer (there may be legal clinics that can provide the information free or at a greatly reduced cost if there's a commonly-accepted boilerplate). "Local" is the key—it doesn't matter what works in another country or state, it has to work where you are.
