What sort of crates are they? I looked at some pictures of shipping containers to show you how you can shade them so they read better.
Notice how the sides are darker than the top. This helps to show that the two sides are angled differently. Secondly, the base color is uniform on each of the sides, which makes them look flat as they should be. Once I had that established, I started adding the details. I've used brighter colors for the details on the top than on the sides to preserve the difference in brightness between the two sides. Once I'd put the details in, I mixed in some other colors for variation. I haven't used the darkest colors much, just for shading in the darkest parts and for emphasizing holes and sharp corners.
Hope that helps.