A smile and a hand shake works pretty good for me. Downside to a contract is you are asking someone who doesn't even know you to give up a certain percentage of what may be there. Most people don't trust contracts, let alone ones they're not REQUIRED to sign. Therefore, when you produce one, they're more likely not to trust you.... just my two cents.
If, after doing your research and finding there is a GREAT possibility of a good cache or something, then a contract might not be a bad idea. General searching, a contract may be detrimental to your amount of sites to hunt.
You should draw one up depending on what you have mutually agreed to as far as percentages go, and it would be a good idea to include a "no liability" phrase in case you get hurt on their property. This'll help them trust you a little more.
If you do still feel the need, keep it simple and expect a lot more "No's".