I get a signed contract for the entire job, then present what we call a phase bill.
Customers really like phase billing. When a room is complete, we do a walk-through for final inspection and collect the total for that room.
It also provides a good working relationship with you and the customer. If they are not completely happy with a detail, it provides an opportunty to fix and not repeat it throughout the house.
Phase painting always worked for me. After my first 5 years in the painting business, I quit asking for a down payment because I was no longer working for strangers.
BTW, a verbal will produce a lot of "I thought you said, I thought you were going to" and more..
Now, over the years I have developed a close-knit relationship with some customers. They would call and tell my office manager to come out and do the job. For them, I would work in the "danger zone" and was never disappointed.
Michael