I am currently helping out one of my consulting clients.
They have started a new division, and have yet to figure exactly where their market is. However, they have more requests for estimates than you can shake a stick at.
In the process of trying to sort the business out for them, they have asked me to step in as a commission sales rep. To see first hand what is happening and to help them figure their systems out.
The moment that I am giving the proposal and I get a verbal agreement from the customer to get the project done. I ask for the deposit. Because the product is generally special order, we are requesting a minimum of 25% down. 5 sales in the last 5 days and no problem collecting the deposit immediately.
If you don't have the financial backing to carry the customer indefinitely, you need to make sure you don't finance the project at all. It is the customers project, let them pay. I believe in using terms like 10% on signing (although, many are asking for much more), 40% on materials arrival, 30% on completion of rough-in, 20% on completion. Or, some variation.
Just my thoughts.