How Do I Make Sure I Get Payed For Birddoging?

I have been trying to get birddogs to find investment property's for me for quite some time now, they have come up empty. I have come to the conclusion that I am better off looking for property's on my own. However the birddogs that I have been in contact with have provided me with a wealth of information and resources that have been very helpful. I have been able to come up with some very good leeds so far. In fact I have come up with more leeds on property's than I can use. After placing an ad for investors looking to buy investment property's in a couple of local papers, and on the internet. The response has been overwhelming. How do I make sure I get payed by the investor for hooking them up with the seller? do I try to get my fee up front? do I have the investor sign a contract promising to pay me if he closes? how do I insure I am geting payed for my service?