Because Steampunk is set in the time of the steam engine and the Industrial era, a lot of the fashion associated with it comes from the Victorian, early 1900's era. It's capes, cravats, and ruffles mixed with gears, brass goggles, and some leather cuffs. The end product really looks like something out of a sci-fi movie but at an anime convention, it's right at home.
The most notable detail of Steampunk is the addition of industrial technology to the clothing itself. If you think about where fashion is going, a lot of our clothing is technology-ready in the some way - there are jackets made with Bluetooth and mp3 players in mind.
Also, looking at this past Fall 2009, metal and chains adorned everything from shirts to boots to the side of our legs. Even if Steampunk seems alien and weird, its influence cannot be missed.
Victoria's Secret 2009 Show
If you're interested in the Steampunk style, the best place to find items is on Etsy. You can also watch the video below for the Steampunk atmosphere and check out the Steampunk Tribune blog.