Closets typically have a hanging rod with a single shelf above it. This leaves several feet of wasted space near the ceiling that could be used more effectively for storage.
While you could add cleats to the wall and another long shelf, a better solution is to make two side by side freestanding shelves out of 1” x 12” pine.
Start by cutting two pieces slightly less than half the width of the closet.
Next cut four pieces of shelving 8” long (or longer if your prefer). Attach one piece to each end of the shelves with screws.
Put the freestanding shelves next to each other on top of the existing shelf to double your storage space.