Deposits: I require the full amount ahead of time if the project is easily imaginable by both parties (as in, "I want this, but in this color.")
If it's an experimental-type order (like trying to match another item, or create something entirely new), I make it first and send photos and let the shopper decide whether to follow through with the purchase, and just list it if they either don't respond or don't want to buy.
Maybe not the most efficient policy, but it's been good for me. I haven't yet had to beg anyone to make their purchase :) or list a custom-order-gone-sour.