This topic is on my mind because of
1) the cold calls I've been doing
2) Jane Kennedy Sutton's recent blog on book signing venues
3) a library event I did just the other day with a panel of mystery authors (that's Dana Fredsti sitting next to me in the photo below. The other panelists were Mark Coggins and Margaret Grace)

I'm also going to recommend the book Intent to Sell: Marketing the Genre Novel by Jeffrey Marks. He has a chapter on "Finding Your Niche" that has some great suggestions. In addition, Marks' Murder Must Advertise website also has a page called Fresh Ideas where authors offer marketing suggestions from booksigning tips to podcasting and an entire little subsection on Twittering.
So, cozy "craft" mystery authors frequent craft shows. Authors of mysteries featuring canines "dog" specialty magazines and dog shows. Historical authors might consider historical societies and genealogy groups. Personally, I've often thought I should check out the Single Action Shooting Society (SASS), what with the "Old West" aspects of my mystery series.
Others thoughts? Anything that you've tried that's worked well for you? Venues are only limited by your imagination!