Usually you go to a community specific for whatever fandom/pairing you're writing, and put up a post asking if anyone would be willing to beta for you. Be careful to read the community userinfo, though, because some communities don't allow that.

Just out of curiosity, what exactly do you need beta'd? :)

A beta is always a good thing to have, even for shorter pieces. :) They help catch mistakes, and can let you know if you're characterisation is off or you have plot holes, etc.

I can probably beta your work if you decide you would like/need one. I'd need a warning a day or so in advance, but I've generally got the time to give things a quick read-through and edit mistakes/make suggestions. :)

There's a harry potter beta-ing community, hp_betas.
You're welcome to post there, or send fics to me!

I beta for lilysunshine1 and stick_around, as well as The Broom Cupboard archive.

Let me know.

*is excited*

Coolies! I can hardly wait!

Either by asking someone you know, or by posting about it in one of the HP communities (hp_slash, the_pimp_cane, etc) (or in your own jorunal).

Also, if you like, I wouldn't mind? I've beta'd friends' fics for a while, although nothing on a continual basis.

You seem pretty well stocked already, but I'm around to beta too if you ever need it. failing_lights(at)yahoo(dot)com. Haven't been doing it much lately, but I think I still remember how it's done. . .