The key to building a good dieselpunk project is education. Learning about the military, architectural and artistic styles prevalent during the 1920s to 1950s is key to designing a compelling piece of dieselpunk art. There are many artists who create sketches, drawings and paintings that evoke these styles that you could categorize as dieselpunk art. Some combine the styles of the past with today's technology while other artists strive to recreate the style itself without applying it to the modern world.
Dieselpunk costumes tend to be less reliant on gadgets and gizmos than those you'll find in the steampunk movement. A steampunk outfit usually starts off as a costume that would fit in the Victorian era. Steampunk costumes often incorporate bulky contraptions made out of copper and brass. They may include goggles, masks and gauntlets. By contrast, a dieselpunk outfit may simply be one that you could imagine seeing in person if you were alive during the World War II era. Dieselpunk costumers might look like extras from a film version of "The Great Gatsby" or military officers preparing for a briefing.
Dieselpunk goes beyond the visual. There are bands that play music inspired by the World War II era whose work you can categorize as dieselpunk. Swing music, boogie-woogie music and close harmony singing are all examples of music styles popular in the dieselpunk movement. Groups like the Puppini Sisters, who use close harmony singing to cover modern songs as well as tunes from the early 20th century, share artistic interests with dieselpunk artists. Other bands align themselves more closely with dieselpunk, combining early 20th-century musical styles with modern ones.
Will dieselpunk ever be as popular as its cyberpunk and steampunk cousins or will it remain a niche subgenre followed by a relatively small audience? What do you think?