"A quick note to say that I am really disappointed that you have still not paid me the [..] that I'm owed for the work I did for you. In the grand scheme of things, it's not a lot of money, but actually I think that this is more a matter of respect.
I have always been there at the drop of a hat to sort out any problems you might have. I love working with you and have always taken our relationship as more than just client/supplier. But if I can't rely on you paying promptly or it takes several attempts and reminders to get my money, that's not good business.
I'm not saying that I don't want to work with you, I do. But if I don't receive what I am owed within the next couple of days, I will have to think twice about helping you in the future."
Never hesitate to ask what's rightfully yours.