This may be the only time that I offer fashion advice on the blog.
Really, the most important thing about a backpacker's wardrobe is that you don't have one. Clothes should be durable, easily washable, and wearable for days on end in any context, from city to jungle. It's also good not to stand out too much; I personally avoid wearing clothing with a lot of writing on it, and stick to neutral colors. (This was only ever a real issue during the red-shirt protests in Bangkok). A lot of the "travel clothing" out there isn't really necessary; it'll just make you look goofy and remind everyone that you're a tourist. My one particular bête noire among ordinary clothes is jeans; they're heavy, bulky, don't wash or compress easily, and aren't terribly comfortable in most climates.
There's other things you can pick up while on the road. I've gotten at least $10 of use out of a 50-cent pair of sandals that I bought at Walmart in Nanjing, China.
Still, there's a few pieces of travel clothing that can improve your trip quite a bit. The Tilley Walking Socks have held up to their "no-hole" promise over the past few years, are breathable enough to wear in the tropics (not to mention warm enough for winter in Liechtenstein), and dry out overnight.
That's fine for when you're wearing ordinary shoes -- and I've found my Clarks Desert Boots to be useful everywhere, by the way -- but what if you want to get out and exercise? Rather than carrying a bulky pair of sneakers, consider barefoot running shoes like the Vibram FiveFingers KSO. My pair went running on the beach in Bali, bouldering in Ko Chang, and on the treadmill in Bangkok. They're also good when you want to protect your feet while swimming in areas that might have sharp rocks.
And, one more suggestion: get some travel underwear. You can go around the world with two pairs of ExOfficio boxers; they wash and wring out easily, and are pretty good swimwear in a pinch, too.
I hesitate to add the Tilley Hat, as some would say that it's stereotypical travel gear that makes you look goofy. I think it looks pretty good, and it's unbeatable for durability and value. You will need a hat to block the sun and rain in the tropics. Mine even went up Mount Fuji and kept the driving rain off of my face in near-freezing conditions.