The above are not the outfits from the show, but just a cast photo that I liked.
These are more of the classic outfits from the show. I would dress up like Malcom's right-hand girl Zoe (the one with the pretty curly hair on the right). Maybe I will some day.
Photos from My Disguises
This is a classic Kaylee look that many people seem to replicate at different conventions.
The Costumer's Guide to Movie Costumes has a great page on how to get different looks depending on the character you're wanting to dress up as.
This is from the page of fashion designer for Firefly, Shawna.