I usually give a schedule at the start of projects explaining what will be done and when, e.g.
Week 1 and 2: Design
Week 3: XHTML/CSS Chop
Week 4: Site Setup
Week 5: Inserting Content
Client has until Week 4 to get content ready and if they don't, project is paid up to that point and it goes on hold. Most clients are fairly good at getting content for a certain date but it does happen that it can't be gotten on time.
Project simply goes on hold and you revise the dates.