Originally Posted by burnsk
If the shelves seem weak at all, you could put a brace in the middle of each one. Just a simple piece of wood (painted white of course) and place them directly above the lower one so the lower one will support the next. It works, I know, because I had to do that to a bookcase that was full (of all things) of books.
If as burnsk said that the shelves are flimsy get some 1/4'' square tube in metal and have it put around the back and sides of each shelf, this works great, I have 1/2inch on mine because it has a lot more weight on each of the shelves.