I’ve discovered I can almost never do business with friends or become friends with previously non-friend clients. It becomes too often an excuse not to pay me for my work.

Absolutely true. I always say no to those arrangements. If I'm going to do something for a friend or family member it's must be relatively easy and free. I'll never do anything complicated enough to charge for it because it gets weird.

just be clear about those lines at the very first moment.

there is a refrain in spanish that says:

If you don't yell, god does not listen

Well, the problem then becomes defining the boundaries. How do you draw the line between work and camaraderie that naturally comes out of a successful client/vendor relationship? You don’t want to be “wooden” with people but you don’t want to be overly-chummy to the point that they feel comfortable asking for free stuff.