The first step to working less is being in a comfortable enough financial position that you can push your boundaries without worrying that if it backfires you'll be out of work and behind on your bills.

Once you have the money to cushion the blow, you can start pushing back on demands for your time with a lot more confidence. A thing I've personally found is that my employer just wants results and things done on time and when needed. They don't particularly care how much time I'm physically in the office working.