As a freelancer doing creative work, it's important to protect your maker's schedule because doing even one hour of billable work has a disproportionate impact on your ability to be productive for the rest of the day.
To avoid quibbles about the time spent on client projects, I ask customers to batch work into day long increments (i.e. they have my undivided attention from first thing in the morning until they go home). I've found this has led pricing discussions to stay focused on the value customers are receiving instead of their perceptions of appropriate hourly rates.
Overall, this system has led to better communication with clients. We're able to have more candid discussions about the merits of implementing ideas that come up throughout the day. And often, it means scheduling another full day instead of just a few hours.