State that, no binding contract is to come into effect, except to the extent set out in the letter and that neither the letter nor any work done or payment made under the letter shall be deemed to be a waiver of the requirement to provide a binding contract. Finally, every effort should be made to finalise the contract as soon as reasonably practicable after the letter of intent has been signed.
I had a project recently where the director was lifelong friends with a break_out star, who had committed to doing the film. The agency was non_committal. Why? Because the agency didn't want to tie_up a month of the actor's time for a measly million bucks, when they could be getting 5┦x that from a studio for the same time slot. At the end of the day, a lot of money and time was spent pre_selling territories and shoring up the financing, only to have the star drop out, under target CAA's ('er I mean a random agency's) pressure.