cpolk, the drawback I see with that, is the way psychics then go on to claim success or at least contribution to the case, no matter what really happened - e.g. they'll add to their web site "Worked closely with Bedrock P.D. in missing Pebbles case," when not one of the cops ever passed word one with them.
You would have to add a clause of "absolutely no disclosure to the press" or something like that. You know, maybe that would be the fastest way to get rid of them? Only let them on the case if they sign a contract stating they won't say anything to any media whatsoever?
Trust me, the fastest way to get rid of them is to say that they won't get paid until results are confirmed.
A contract alone would prevent them from libel or slander, such as claiming undue credit. You will have a clear definition of what they were to provide and whether or not they provided it. If they do provide it, let them shout it from the hilltops. If they don't, threatening legal action will remove it from their website.
That is the mistake that police departments make. They don't enforce signing contracts because they believe the nonsense about "you must forgo common sense", and as a result have no legal standing, because they cannot prove that the psychics did not provide the results they claimed that they would.
For instance, without a contract detailing what type of information counts as viable information, a psychic could say, "I see a body of water" on a case involving a person who went missing on a fishing trip, and they would legally have the right to proclaim that they provided information.
Without a contract, that is paramount to having to pay an archetitect's full fee for building a house because he gave you the number of a guy who sells lumber cheap, then having the archiect proclaim that he took a part in designing your building. With a contract, you can prove what was supposed to be done, and through record-keeping you can prove what was actually done.
On that note, have the "psychics" provide everything in writing and/or on tape.
Or just skip the psychic connection altogether. This is a lot of work on your part, and you should only do it to placate the family members who feel that you are not using all the resources available to you. A statement regarding psychics, as you had posted earlier, is the route I would take.