My employer works on verbal contracts quite often, but he's really good at collecting money for extras and gets paid every time. I'm really bad at collecting money for extras, so I itemize the work in advance.
Your incentive to get good at collecting money is to look at your kids and tell yourself that someone is going to go without, and it won't be your family. Then, go see your customer, look him straight in the eye, and tell him you want your money. If you don't get paid, then go home and explain to your children why their lives aren't better.
Good luck!
