Someone’s put together a sensible guide to how to go about refusing to be searched by Metro Transit Police. The Citizen’s Guide to Refusing DC Metro Searches has a downloadable half-page PDF you can print out and carry with you or give to friends. A disclaimer takes pains to note that the site isn’t providing legal advice, but tips like staying calm, speaking clearly, not running, exiting the station promptly after refusing to be searched and not resisting should you be detained all seem like excellent advice.