What are the best practices for timing scene changes as a stage manager in theatrical productions?

To all of these good points I would add, speaking directly to the actors involved in the upcoming scene. The crew needs to know exactly where each prop is set. The crew also needs to COLLECT the item(s) from the actor(s) if they were instructed to carry them offstage after their scene is completed. All props need to be placed back in the props holding area until needed and organized by scene/time/cues.

Rehearsal. Work with the crew to find the best way of storing, pre-setting and manoeuvring items of scenery, and the ten getting them off and packing them away. If you are doing it to music then tie it to the music where possible. In some musicals extra music was actually written to cover a complex scene change.

Treat the scene changes like any other part of the show. Do your preparation in the planning phase, block (choreograph) them like every other moment, rehearse them and keep asking the team for ways to make them better (better for me is shorter and less noticeable). The rehearsal of the scene changes should be given a respectful amount of time in the schedule to make them work but it also shows the balance of the crew that you respect the work they need to do. Moreover include everyone that needs to be a part of the rehearsal so all elements are worked on - this might involve some cast and the director too.

I found that being able to read music is so helpful, counting the beats - making it all seem like a concerto! Both an acting piece and written music are artistic productions, and this means every moment is balanced and in time!

Review the scene changes after each performance just as you would the performance of the actors and lighting team and sound team and and and ... The review keeps all participants aware that they are important and that they need to do well for the overall show to do well. Make sure you LISTEN to the team as you review them; they might make things easier and better for the show if you listen to new ideas, comments, concern and even come complaints.

Never underestimate the importance of a well executed scene change. It is a very important performance within the show - granted it is a performance that you hope the audience does not notice happening and you most likely need it to be as short as possible but that in itself is part of the show. Make sure you thank, acknowledge and reward the team for doing well. It is so important that they know how well they are doing and that their work is appreciated!

As a director also, my thoare that the director as the first word of advise on fixing the scene changes. They are part of the production and the director should make the needed changes to insure everything is smooth and everyone is safe.