I also subscribe to the idea of simply referring potential guests to others I know in the area, who offer similar accommodation. Being only our second year, I can't say I've done much of it yet, but other LMHers have referred to me and I see no reason why I shouldn't reciprocate.
Keep it simple, is my philosophy. If someone refers to you, you refer back. Far too complicated (and potentially divisive) to be working out commission rates etc and then as pointed out by A2, you could breach the law if you are not very careful.
And if you refer one or two more to them, than you receive in return, well c'est La Vie !
It won't take long for you to figure out if the're not playing ball, will it