Pirate costumes are based roughly on 18th-century garb. Look for leggings of soft cloth and white shirts with billowy sleeves. Add a large sash tied around the waist, hanging to one side, and a belt with a large, square buckle and a pair of boots with wide, roll-down tops (be sure to tuck your pants into the boots). If you don't have boots, cut off your pants just below the knees and go barefoot. A simple button-down vest can be a good stand-in for a waistcoat.
Be sure to use colors and textures that reflect tradition. Brown, black, white, cream and red are good choices. You may also want to dirty and distress your clothes before you wear them. Try soaking them in coffee or strong tea; rubbing clothes with a cheese grater will wear and fray them on the edges.
It's almost impossible to have too many accessories. Wear a bandanna around your head if you don't have a good pirate hat. Attach a single-hoop earring to the bandanna at your ear if your ear isn't pierced.
A pirate should always have a sword, so attach a fake one to your belt or make a prop scabbard and sword handle from cardboard and black duct tape or electrical tape. Decorate the sword handle with metallic paint.
Wear as much old-fashioned costume jewelry as you can find. Look for things like gold chains and large, thick rings. Some of the best offerings come from supermarket vending machines.
An eye patch is easy to make from almost any black cloth or flexible material and is a good way to improve your pirate look.
The right makeup and hair will complete your pirate costume. If your hair is long, rat it a bit and decorate it with a few large, old-fashioned beads. If you want to wear a wig, look for a long, straight wig or a dreadlocked wig.
Brush some bronzer on your face to give it a tan look and use black eyeliner. Pencil in some facial hair if you don't have your own, such as a small mustache and goatee, and give yourself a "five o'clock shadow" using a small, rough sponge and some thin, liquid makeup.
Finally, give yourself some pirate tattoos using black grease paint. Think of stereotypical pirate symbols, such as the skull and crossbones, or nautical symbols like a compass, anchor or ship.