I don't do this kind of work anymore but I used to charge 40% at the start, 30% on delivery of templates and page design, and then 30% on completion. That tended to sort out who was serious and who not.
Plus, yes potential clients can seem lazy, except when they suddenly decide they want it and they want it yesterday! This usually of course coincides with you working on another important client. I think that is called sod's law!
And that was usual for clients of mine, yes they want to do the work with you, yes they will get started soon, delay and delay and eventually up to 12 months later it is all of a sudden blindingly urgent, it has to be done yesterday, it does not matter what else you are doing at the time!