I would suggest, if you want a good beta, to find someone whose writing you do like, someone whose work you enjoy. Try and establish a connection, either via reviews and replies, or even a forum PM or email. Get to know them a bit. There's a lot more to a good beta than just skill. Personalities have to match to a degree as well.
You also need to decide what it is you want from a beta. Is it someone who will give you solely grammar and technical assistance, or do you want someone who will be honest and tell you when something isn't working for them? If you write original work, you don't need to worry so much about whether or not they know your fandom, but do they grasp your world? Does your setting make sense for them? If you write fantasy and your beta is not a fan of fantasy settings, you already have an issue since they can't get immersed in your world.
Then there's the dreaded real life issues. If your beta has a family or is in school and is subject to papers and finals, real life is going to intervene from time to time. Nothing is more irritating than waiting for your work to return, but on the other hand, it's equally annoying to feel like your real life is supposed to take second place to the free services you offer as a beta. This is also where having a good rapport comes into play.
It's not so terribly dire. You can find decent beta readers, but it does take more than leaving a post on a thread. Like anything else worth having, you need to work at it a bit. Just don't give up.