Appeal to their hearts as well as their brains. Find different ways. Report back on the success during the funding period, offering highlights. Have volunteers or participants write letters of thanks. Give them a certificate of appreciation or plaque they can frame.
Nothing is as powerful for givers as to be thanked by their own employees for making a gift.
You may find that developing a corporate partnership is easier than I have outlined here. You would be lucky if that were the case. But if you follow these guidelines, you'll be in a better position to realize the kind of partnership that keeps on giving.