First, Amanda Fitch of Amaranth Games (http://www.amaranthia.com) posted her guide to selling indie games:
This is a nice, short, nuts and bolts guide to the details of actually selling your game once you've made it, with a short section on Beta testing.
Next, Jeff Vogel (Spiderweb Software - http://www.spiderwebsoftware.com/) has a really nice, detailed blog post (on the Bottom Feeder - http://jeff-vogel.blogspot.com/)on running a Beta test:
This is really helpful and worthwhile; the man has done this sort of thing before!
I have two experiences with running a Beta test for an indie game. First, I ran a long and extensive test for my play-by-email game Atlantis (http://www.prankster.com/project/index.htm). I had several hundred people playing, which made for an excellent testing atmosphere, plus two other things really helped. First, as a server-based game, I had complete access to the game as it was going on; I could see what orders the players were issuing, complete reports on their positions, etc. Basically I had all the data (and more) that I could want for doing testing. Second, as a multi-player online game, the players were already used to lots of discussion, with mailing lists, a weekly newsletter, etc. Getting feedback or bug reports wasn't a problem at all; the problem was distilling all the information into something helpful!
My other experience was testing my free SENG demo "To the World Tree" (http://www.prankster.com/ttwt/index.htm). My experience here couldn't have been more different from Atlantis. Since it was just a demo, I did not set up a formal testing program; instead I just released it and asked people to send feedback. This was a mistake; I don't really know how many people played, though it was downloaded thousands of times, who knows who actually played the game? I got a few really detailed feedback forms with lots of helpful information. I got more (but still not a ton) of limited feedback; this feedback could generally be distilled into either "This sucks!" or "Attaboy!". But mostly I just got silence.
So, I have two thoughts going forward as far as testing. The first is definitely to create a small but active community for testing. Only allow the testers access to the game bits (don't just release a demo into the wild), and really strongly try to interact with them (and even get them to interact with each other). I suspect some combination of an announcement mailing list and a forum is the best way to go.
Next, some sort of real, hard data from the game would be nice, too. I don't want to try to get super-high-tech (because that would take a lot of effort), but perhaps the game could periodically offer the player a log file that he could email to me or something. Maybe a web-form would be even easier. This log file wouldn't help much with bug-fixing, but if I logged the right things, I could get a good idea of how the player is approaching the game. Which class he is playing, which quests he is doing at what levels, what monsters are particularly easy or difficult, etc. That kind of detail would be really nice to have, and it is pretty hard to extract from the player directly, without actually watching him play.
Those are my thoughts; if nothing else I really recommend you read Vogel's piece for good solid advice. Depending on my work plans, I may be getting into some sort of testing program this fall.