This post kind of ties into my previous one on the Holt Steam Tank (quite strongly, truth be told), because it focuses on things you need to think about when creating steampunk vehicles. My opinion on making things up when something real will do is well-known, so onto the steampunkery.
Steampunk vehicles are about more than looking pretty, which we can all admit they do very well. They also have to appear functional (the keyword in that phrase being appear of course -- no one's expecting you to actually build an airship or a steampunk motorcycle).
What's this mean? The simplest solution is to adapt real machinery to a steampunk style. Kind of like how the aesthetics of World War II infused the design of Keith Thompson's War Zeppelin:
If you click on the photo and go to the larger version, you'll see the way the design flows. He hasn't shown any exhaust steam, but the engines are probably set at the back near the tail or on the underbelly. Either way, the point is that something like this could exist. That's the major thing when it comes to designing steampunk vehicles.
For CALLARION AT NIGHT, I created a tank nicknamed the Turtle. It's a copper dome on treads with a boiler/exhaust in the rear and a 50-caliber machine gun up on the roof in addition to the main gun. The design was inspired by Crabfu Steamworks' Lobster Tank:
If you want good starting points for steampunk vehicles, you could do worse than perusing the Crabfu website. He also offers a brief tutorial on how to draw steampunk machinery, which gives similar advice to what I've been extolling here: Make it look like it could possibly exist.
Other than that, and a few general steampunk accouterments such as filigreed details, wood paneling, and a variety of other materials in warm tones, the world is your proverbial oyster. But remember, it has to look plausible for it to be believed.
More steampunk art can be found at Dark Roasted Blend.
P.S. Go check out Natalie Whipple's post on Steampunk. The fashion she details is very interesting.