Provide the necessary authority, resources and, support. Delegate for results. Let your staff handle projects in their own way. You are after results, not process. Review progress and follow-up.
Interruptions are huge time-wasters.
At the very least, they break your train of thought. More often than not, you spend precious minutes on something that is not important or urgent. Discourage people from dropping in without an appointment to chat except at appointed times. People are free to drop in then without an appointment. When someone comes in your office at another time, politely ask the purpose of the visit.