In the past I've kept mine in boxes under a workbench, or otherwise hidden away, and a consequence has been that I've ended up buying a piece of glass for a job for which I've already got something suitable, and I then end up with loads of quite expensive glass that I might not use.
I had an old bookcase in my garage which has come to the rescue. The bottom shelf is big enough to take an A4 file without any of it protruding; a coat of white paint and suddenly it started to look the part.
On the top two shelves there's plenty of space for tubs of cement, whiting, and containers of small offcuts of glass. The bottom shelf is ideal for the larger sheets, but I had a problem - how to keep them in a way that that allowed me to see what I had.
A visit to Wilko's solved that: these magazine files are available at £2.75 each, and have holes to allow them to be hooked onto a pair of screws - ideal to stop the file toppling over and breaking the glass.
I fitted screws to the back of the bookcase and, for the time being at least, I have enough storage for my glass! I bought five of the magazine files to start with, and have since got another three; there might be space for one more should I need it.