As far as venues are concerned, I've basically been exclusive to the bar/restaurant style venue. But, that mostly comes from my background as a karaoke DJ. I've hosted Rock Band nights at sports bars, Irish pubs, pizzerias, and even a bar designed exclusive around video games. I've also gotten into hosting competitions for charity nights and things of that nature.
Basically, a "good" venue is one where your presence there will equate to a profit increase for the venue. Typically, bars are the ideal choice since your purpose is to encourage patrons to come and presumably spend their money. Of course, since a large part of the Rock Band demographic is under 21, finding an all-ages venue is something serious to consider; maybe bowling alleys, teen clubs ("juice bars"), pizza places, and restaurants. I'd stay away from national chains, since usually they have a policy about outside entertainment that comes from the corporate level (though franchisees sometimes have more latitude in that regard).
Local fests might be a great way to promote your own brand, but you have to consider how "everyone" wins. You'll probably have to pay to secure a booth, and from there, what exactly are you promoting that will result in you making money? Being sponsored by a local business is a good way to get started there, even if it's not a venue (consider an electronics store or something similar).
If you're just starting out (assuming you have all your equipment resolved), the first step is to get your foot in the door somewhere, anywhere, that will let you start building a fan base. You may have to make concessions to secure that first "gig", but once you're established, finding gigs is a lot easier. Remember that you are in charge of your own destiny here, so be willing to negotiate and work on your salesmanship skills.
As far as a trade group, I'm willing to jump on the initial organization, though at some point I'd like a few other businesses to help manage things. I'm envisioning a group that agrees to certain standards of practice, is available to each other for advice and tips, and is willing to collaborate across markets to promote the good of the group. I'd LOVE to get approval from Harmonix once this organization gets started; that way, we could market ourselves as members of the "Official Organization for blah blah blah Rock Band". You'd be surprised how much a little sense of legitimacy helps at the negotiating table.