Just about everyone owns a Hawaiian shirt. Add some Bermuda shorts, a visor, a camera (or two) around your neck, and sandals on your feet (socks optional). For a funny effect, stuff the butt of your pants for that extra-dorky look. Find or make some brochures and tickets to stuff in your pockets and you're good to go!
Wear your funkiest colors. Girls: layer two or three skirts on top of each other, with a peasant blouse and lots of jewelry. Guys: wear loose pants tucked into boots, with a shirt unbuttoned to reveal several gold necklaces (plus a vest if you have one). Wear a scarf around your head and another around your waist, and boots on your feet (or just go barefoot).
Start with a bandana around your head. Boots are very helpful but you can make boot tops by wrapping dark fabric around your lower leg flapped over at the top. Women--wear a skirt with a ragged bottom. Men--either tuck some loose pants into your boots, or cut the bottom into a ragged edge. Wear a loose shirt unbuttoned at the top, with a vest (it's easy to make a vest by cutting up an old shirt). Wrap a piece of fabric around your waist for a belt.
Everyone owns jeans, and chances are good you own some kind of western or plaid shirt and boots. If you can get your hands on a cowboy hat, that's great. Accessories like a gun belt, lasso, bandana around your neck, chaps or vest add to the look.
Easiest ever. Just wear your clothes a size too small, with your shirt buttoned all the way up, and pants pulled up too high, over white socks and Grandpa shoes. Add a pocket protector, fake zits, and gel in your hair to make it look extra-geeky.