"A partnership is a two way street where both parties give and both parties receive. There are expectations on both ends and both parties have a vested interest in one another’s success. Everyone has something tangible to lose and something tangible to gain." -Yost Autosport
Partnership is about exchanging value between yourself and a company. The first question you ask yourself before emailing out a proposal is "what can I do that they can't?" You have to offer something that the company itself is in need of, or could additionally profit from, and can't get without your help.