Have enough personnel. If the scene changes are minor, one or two stagehands can probably handle it. But if an entire stage set needs to be changed in just a few minutes, make sure there are enough people to move the old scenery out and the new scenery in as quickly, quietly and safely as possible.
Add mechanical aids. Something as simple as wheels on heavy items can make the changing of scenery much easier and require less personnel. If your theater has a fly gallery, consider rigging large, two-dimensional pieces to it so they can "fly" up out of sight or down onto the stage in just a few seconds.
Practice. Just as the scenes and curtain call need to be rehearsed, so does changing the scenery. Make sure the backstage personnel have time to practice the scene changes so the audience sees a smooth, quick transition from one location in the play to another.