I know it's a chicken & the egg problem, but demonstration of actual 'entrepreneurship' is my suggestion. The best way would be to get a demo or product working ASAP and get users, fast. You have a better chance of arguing a case with something more concrete than a prospect.

If this has to be done while you work in another job, the 'Schachter' model (work, expend little & build a product before a company) could be the go.