Whats the best way to avoid being cheap while being nice in a professional environment?

This is a problem for me because I do what I love. I like working and I would do it for free, but I need to charge to make ends meet. However, when a project just keeps extending, although it is still fun, I need to ask for more money. That leaves me feeling like an ass, but I have to do it. Have you experienced anything similar? how do you deal with it?