If you're like me, you started stage managing early on and quite by accident for a school or theatre company that was desperate to have a stage manager. You were pulled in from another theatre discipline, or brought into it cold. This means that unlike other areas of theatre where you generally start at the bottom of the ladder and work your way up, stage managers often join the professional arena with little or no experience in the lower positions of the field. Therefore, it's a good idea when you're moving into the "real world" (or ideally beforehand!) to get some experience as an assistant stage manager or a member of a stage management team. It will give you an idea of how massive the job can be, and it will help you to better understand what your assistants will be going through when you get back to ruling the roost.
