I update a whiteboard-based calendar with planned meetings so I can see at a glance what my day and week looks like. It helps me to visually pace myself around scheduled timeslots with customers and internal partners.

Details of the meetings stay off the whiteboard and that keeps it uncluttered.

The size makes it big enough to see from my desk and I can add and erase on the fly.
I also sometimes add date critical deadlines for various projects.

And it allows others to quickly see my commitments are for the day if they are walk-ins to my office.