How to mix business with my girlfriend?

I need her help. I will pay her, not as an employee, e.g. hourly wage. I will give her a share of the income. I hear not to mix relationships and business but i see many people who are married and do business, and do it successfully! I want to be like this with her. She is my special lady :-) How can we avoid the 'shaky ground' "don't mix business with pleasure" any advice especially from people who do business with their wives girlfriends AND are happily together! Thanks everyone!