OK here goes based on what my wife told me. Now keep in mind she ran a couple of sandwich shops as well as a couple of luncheonettes at golf courses through the years.
Keep the menu simple. Keep 3 types of bread on hand, white, whole wheat and rye. Buy your meats like ham, corn beef etc in deli amounts so they can be cut fresh for each sandwich as needed. Serve a single soup as a daily special only, offer a second soup as the house special. Keep cole slaw, potato salad, chili, egg salad and macaroni salad on hand fresh daily. Serve chicken, tuna, shrimp salads with fresh lettuce and tomatoes when possible. Have beverages that your clientèle wants be it sodas, ice tea, coffee or beer (if possible). Prepare only what you must, doing as much as possible on a per order basis.
Now that the menu is done her other advise it to know the daily volume coming through the door and plan accordingly. She told me the real secret between those that make it and those that don't is the food and judging the daily volume on a given day being correct. The latter is if you prepare more food than needed you lose money in waste, so figure it out early and stick with it. Running out isn't as serious in the long run as having too much.