I hear this a lot, but I'm not sure how to implement it. Some people have suggested that freelancers ought to bill for the entire project, but in doing so you run the risk of losing money by underestimating the amount of work and time involved. Some have also suggested billing daily instead of hourly, but if the client expects a "day" to mean "8 hours", how is that any different than just billing an hourly rate?
How do you handle your billing?