There is no One True Way to organize your committee into departments. Often times a convention will run for a few years one way, and then combine departments that share a lot of the same resources or purpose into a single department. Or a department may split, as the needs of the convention grow. Do what works for you, and recruit reliable department heads. Create, publish, and maintain a clear set of objectives and methods to document continuity of what works, what doesn't, and why. Check on the senior staff regularly to make sure they're getting whatever support they need from you and the rest of the committee, pre-con and at-con. Department heads then recruit what staff and at-con volunteers they need to accomplish the goals of the department.
Have your department heads document the procedures of running their department, and train people under them so that you have a pool of people ready to be future department heads, and you are capturing knowledge from one year to the next.
A common way to split a science fiction convention into departments is like so:
You can easily see how Volunteers might also go under Operations, Masquerade and Dance under Programming, etc. A small enough convention may not have a person dedicated to publicity separate from their publications head, or an information desk, or whatever. And of course, some conventions don't have Art Shows, or Charity Auctions, or whatever. Try to pick a structure that best supports what you do and how you want to do it.