That's exactly what I do for long term clients with open ended projects. I use Freshbooks' timer to track my time. Whatever that totals to is what gets billed.

That's exactly what I do for long term clients with open ended projects. I use Freshbooks' timer to track my time. Whatever that totals to is what gets billed. For close-ended projects I generally quote up front and track my time for my own purposes. For scope changes and other additional expenses I often use a timer and bill actual time.