Try one of these two simple fixes to take out the sag.

The easiest option is to tilt a piece of wood cut to the height of the shelf into place to prop the shelf back to it's original height. This will cut the long shelf into smaller shelves. This gives a ...

plywood, as the surface the devices sit on, would help. the thicker the better. something like 3/4 inch would probably work. you could also "strong back" the shelf, underneath, with a steel cross ...

What compares to the annoyance of drooping drawers? How about sagging shelves? A shelf isn't likely to sag without the weight of some type of load being placed upon it -- too much clothing, too many ...