What are the best practices for partnering up with people and other companies for brand awareness and marketing each other?

Have a fitness training company which has built quite a reputation due to name clients. I would like to branch off and allow people I know to use the brand and market as being affiliated with us. In return they would pay a monthly fee. What do I need to do legally? What needs to be done to protect myself and the company? Would I be able to do this with just a contract agreement and a waiver that their clients would sign protecting us? What about tax implications? Could this just be counted as ordinary income?