We had a successful yard sale back when Columbia Heights had a neighborhood-wide yard sale. It was awesome–so many people had yard sales, and that attracted a lot of people to just walk up and down streets. So if you have a neighborhood group, maybe see if any of your neighbors want to join forces. At the very least, you all can check out each other’s sales. Also, I suggest starting early. Some of these professional yard sale types were buying stuff from me when I was still setting up at 7:00 am. Display stuff so that people can see it from the street too–some people seem to be shy about walking up onto someone’s lawn right at first. Finally, make it fun! Have fun signs to direct people, play some music, and hang out with a beer or something.
Also, advertise in the forum here on PoP!