Officially: once we arrived in a city where we were performing a last-minute show and none of the local papers or radio stations had received our press packets, so we had no pre-sales at all for our show. We decided to go out ourselves, like artists at a fringe festival, and advertise with word of mouth. We printed 4"-6" flyers and literally walked all over downtown, being friendly and interesting and handing out flyers. We also put flyers in shop windows and ear registers, if the shop gave us permission. We ended up selling out the show!
Unofficially: when we are out and about in a city where we will be performing, if someone asks to see an acrobatic trick, we almost always do something for them and tell them about the show. Depending on where we are, how old they are, how inebriated they are and how far in the future the show is scheduled, doing a trick seems to get 1 to 5 people out of every 10 to come see a show.