I had a family start a few months ago. Mom is a teacher, Dad's work can be seasonal. She approached me the first day and said that things were a little tight, blah blah blah, and could she pay up on such and such date. I grudgingly said yes. I am certain she could tell that I didn't love it, and I told her that it was not something I could do on a regular basis.
In the last couple months, they have paid late, paid less than the agreed upon amount, and so on. Last month I ended up telling her "You need to figure out what works for your budget in terms of how much and when (monthly, weekly, bi-weekly, etc). I am willing to work with you, but I cannot accept what-you-can-when-you-can."
That was their last strike, and if we should have any further issues, it will be the end of our business dealings.