I had a teacher when in art school who said he billed in increments. He'd charge so much for preliminary work/roughs, then based on the amount of work he broke the work down in stages and required payment for each stage before moving on to the next. If the client liked the work they paid. If they chose to back out he was paid to the point they backed out. All this was agreed upon in a written contract he'd present at the beginning of the job. He said that if the company was sincere they had a happy marriage. The shysters weeded themselves out when they complained that the arrangement wasn't fair and wanted to pay when the job was completely delivered. He found that this worked for him.