Just lay the caps face down on a sterile piece of paper or aluminum foil. You will find that a paper taken from the center of a book or ream will be pretty damn clean. cover the cap with an overturned bowl or glass so that errant breezes don't blow contaminants under the cap, or disturb the pattern (they look kind of cool), then fold them in half and wrap in foil, put this in a baggie when you are sure that the paper is dry (it picks up moisture from the cap) - that's it - store in a dry place out of sunlight.