Shows can certainly be a good venue for portraiture sales, but the key to show success is - getting into the right "kind" of show.
I have been doing shows for about the last three years and the thing that I have discovered is that there are a couple of kinds of shows:
1.
Shows that are more "craft" than "art" - country kitsch, soap doilies, ceramics, things to stick in your yard. Avoid these shows like the plague - you will spend your time singing "one of these things is not like the others...." and you won't sell much because people are there to buy crafts - not fine art.
2.
Shows that are "huge" - Some shows are so large that art and craft does not matter so much. I am doing a Holiday Food and Gift show in Denver in November where they get more than 40,000 people over 4 days each paying $10 to get in. It is a huge show and people come there to buy. You find all sorts of things at this show and fine art is well placed.
3.
Shows that are for "art" - Pure fine art shows that are juried are great to get into.
Finally, you have to define what you are in the show to do - sell prints, advertise your portraiture, etc. I do shows to advertise. I have a few prints in a small bin, but my primary purpose in doing the show is to get people interested in my work and thinking about having me do a portrait for their family. So I have cards and signs and portfolios everywhere, and as many originals as I can hang. I try to always work on a charcoal while at the show (charcoal is easier to do at a show than actual painting)- working is a great way to bring people into your booth.
Hope that helps!