After a month of driving uber, you will receive an email from uber with title, "Earn MORE CASH with Partner Promo Codes". I believe this is one of the best way to make extra money. You can print our your business cards during or after the completion of your ride. I recommend after the ride because you don't want to make your rider uncomfortable. You don't need to oversell them. You can just tell them assertively that "Share it with your friends, your friends can get their first ride for free" while you hand them the cards. That way, they have no way to reject it!