Creating Strategic Partnerships?

So one of my ideas to initially start acquiring new clients for my corporate event planning company is to attempt to create some strategic partnerships with some marketing firms in my area. I`ve done some research and gotten a list of marketing and PR firms in my area who appear to be on the smaller side and who don`t advertise event planning as one of their main services (at least not on their website) hoping they would be more receptive to outsourcing their event planning. My thought was to approach these firms and offer a mutually beneficial arrangement for us both, by independently contracting me to do their event planning for their clients. They can tell clients they offer the option of event planning as a service without having to hire another full time employee. The firm wouldn`t have to pay anything (it`s just an additional charge for the client), and it frees up their people to work with other clients and projects instead dealing with the logistics of an event. Meanwhile, I would have a more diverse group of business coming to me (as opposed to me hunting them down) on a more regular basis.
I know there are many marketers out there on SuN so i`m hoping to get some advice on how to best approach these companies.  What would sell you on the idea of forming a strategic partnership and make you want to work with me? Why would you NOT want to partner with an independent event planner? Would you want signed contracts of partnership or just an informal agreement that you will contact me if you have the need? Is this idea good at all, or should i be targeting the individual businesses instead of a middle man?