As great as milk crates are, they still, in essence, are just open boxes. They tend to stack in the wrong way to store anything other than milk -- they interlock in such a way as to block access to the crates below, forcing you to break down the whole stack to get at anything at the bottom. To add a little utility to your crate collection, all you need are some industrial-strength drywall anchors, screws, and washers. By orienting the crates on a 45° bias, they turn into a self-supporting structural system while adding even more storage space in the "Vs" created by the vertexes.
If you just bolted milk crates to the walls at normal ninety-degree angles, each one would bear only on the one directly beneath it. By rotating them, you've created a situation where each crate bears on two crates beneath, a much stronger, holistic system. And, there's a nice aesthetic twist.
This project couldn't be simpler. It literally takes minutes to take up, minutes to take down, and leaves nothing but a few holes to be fixed up with spackle when you move on. The materials are (mostly) free, if you know where to look -- I'm not condoning stealing milk crates! They are legally available through many channels, including just asking shopkeepers instead of running off with them. I got mine through a combination of inheritance and combing through dumpsters and studio buildings at the end of the college school year.
You will need these materials:
As many milk crates as you see fit
3" drywall screws
Cone-shaped aluminum drywall anchors (50 lb. weight rating)
Washers of various sizes
You will need these tools:
1/8" or 1/4" drill bit, depending on drywall anchor brand
Philips head bit
Speed square or similar