This is an interesting subject and i look forward to others responses.
I ask for a $500 deposit to get on my wait list- this immediately separates the time wasters from real customers. In the past, i've traded many emails back and forth with potential customers about custom ideas then never hear from them again. Now I limit the initial 1001 emails to a few good emails with ideas, then get a deposit
before turning my mind to any specific custom creative thinking, which is the most difficult element for an artist.
When I get around to starting to gather materials for the build, i ask for the first of usually 3 installments-
1st- Beginning the build
2nd- Just prior to finishing, and
3rd The total remainder prior to shipping to the new owner.
I kinda leave it up to the customer to decide how much their installments will be (according to what they can afford), but i've found even amounts (3, or 5 or whatever) spread out is the best for my own book keeping.
I've had people pay everything at once which is nice until you spend it all way to soon