just have a look at" full moon party" on google images & you'll get the idea.
Seriously anything day glow, psychedelic, hindu imagery,anthing to do with UFO's, aliens etc on t-shirts & some hippie beads.
Jeans, a tied dyed vest top, flipflops & some beads would be enough. Dayglow face paints are pretty easy to get hold of too - paint something pretty on your 3rd eye! But I strongly believe fancy dress should be the only cirumstances that one dons anything tie-dye, so make sure it REALLY IS fancy dress, otherwise you'll just be a crime against taste.