Or just CAREFULLY wipe the surface with a damp sponge when its bone dry, if your clay has any sand or grog in it, you'll remove a layer of the finest particles and leave just the groggy stuff! This takes a delicate hand, you don't want to wet the piece, just remove the surface layer. Look up 'water etching' for more info.
Like Neil said, just about anything organic can be rolled in/burnished in/pressed in, then either removed or burned out. Other stuff I've used and liked (but it may HAVE to be removed before firing) includes rocks, onion bags, highly textured fabric, rope, stencils, old computer parts, scraps of packing material, old crochet doilies... your imagination is your only limit!