I have a client who was consistently late like this. The solution I had the last time they wanted me to do a project was to require them to pay a percentage of the bill up front before I would begin any work for them. It worked out well, they paid 100% up front without a hassle. I had my check in about 3 days from them, as opposed to the 4 months it took the project before. Another thing to consider is adding a late fee, like a certain % per day, week, month, whatever you choose. Once they see their bill start climbing simply because they haven't paid it should get their asses in gear.