When taking on *any* design job, I clearly state upfront that a 50-75% deposit is required. I also state a time frame for completion and ask the client if they have any specifics that need attention before the project is began.
If they do not tell me upfront, then additional charges apply if modifications need to be made at a later date.
I've worked with probably 100+ clients, not including those that are return business and this has worked out every time except once and I gladly refunded the payment.
I base the amount of work placed into the job based on pay as I do it for a living. If they do not feel my work is worth what I ask, there are many other designers that will provide poor coding, over-sized images and un-optimized work for less

.