Essential-- ASAP, inform the HEALTHCARE-NOW website so they can list your event, and put notices in local papers and shoppers. A few days before the 30th, when everything is firmed up, notify the media--print, radio, local TV or cable news, and appropriate blogs.
Optional--1)see if PNHP can dig you up a local speaker (they have a speakers bureau), or find one from a local organization, and get a small loudspeaker system; 2)offer spots for lit and sign-up tables on your site to groups working on single-payer; 3)find an artsy person to make a couple of signs and have them commercially reproduced for sale or free distribution to participants (depending on finances); 4)issue a press release the morning of the event on the reason for the demo including quotes from leaders of participating groups and articulate members of the general public; 4)create chants and/or topical songs sung to familiar melodies and run off copies of lyrics for participants.
And Have Fun.
Related news: It seems a leader of the lobbying arm of the AFL-CIO was quoted as supporting single-payer:
http://www.pnhp.org/news/2009/may/single_payer_system_.php