I agree with everyone so far in that establishing a good beta/writer relationship is based very much on a mutual appreciation for the same thing. For example, I love KathyB's writing and have asked for her help on one of my stories. She kindly assisted me and her input was invaluable. However, on this last story - high angst and WHAM factor - I think KathyB would have been quite possibly the last person I would have approached for a beta reader because I know she doesn't prefer that kind of story.
But also, I think there is another critical element. I don't think it's only what you write but also how well you work with the betas who have so kindly offered to help you. Because that's exactly what they are doing - helping you. Taking some of their very valuable time to read your story carefully and make remarks as to what they like and how they think your story could be improved. If you aren't happy with how quickly your beta reader can get through your stuff or that they might need to bump your story down in priority for other RL issues, then I think that relationship is doomed to fail. In which case, I see it as only reasonable for the beta to explain his/her limitations and be permitted to walk away from the story without any hard feelings.
Same thing goes with comments - if your beta consistently gives you advice with which you disagree, then it seems an almost pointless exercise to have him/her look over your stuff. Not that you should feel pressured to make every suggested change. Simply, if there is rarely if ever an issue on which you agree, then it seems as if the match is not a very good fit and you should perhaps find a different beta reader. I know as a beta reader that it would be very frustrating to make suggestions that were constantly ignored or that incited actual argument. It's all a matter of opinion and since no one gets paid to be a beta, there is certainly no reason to endure a headache.
Taking care of your beta can be as simple as a thank-you in the notes of your story but should also include a healthy dose of respect for that person. I think a writer should never, never assume that the beta has nothing else to do in his/her life that they can focus entirely on the story at any given moment. Patience is key as is understanding that sometimes things just come up that take precidence. For me, I have a nasty habit of shutting down all other functions when I'm in the midst of writing. I just can't focus on anything else, so I have to apologize profusely to the writers on whose stories I'm working because it takes me much longer to turn my remarks around.
I have two beta readers that I use consistently, and as long as they are willing, I feel very happy and grateful for their help. I do ask them, however, with every new story if they have the time, energy, and inclination to start up a new effort. I warn them if it will be long and involved. And I tell them with every section that I send to them that they should take as much time as necessary. If either of them were to need to back out for any reason (even that they just got sick and tired of the story and no longer felt they had anything of value to offer) I would competely understand. Nothing in this world worse than trying to beta read a story that for whatever reason is just not sitting quite right with you. I'd much rather have someone tell me honestly that they need to beg off than to grimace every time they see my name in their e-mail inbox.
And I return the favor - my door is always open to look over my betas' stuff in return. In fact, that's not a payback - I view that as a bonus for myself because I get to read their amazing stuff first!!
Sometimes I've prevailed upon people to help me out with questions regarding elements of a story. For example, I grilled DocJill one night on IRC for some medical answers. I've engaged Tricia in numerous e-mails regarding things about Australia that I've needed clarified. And I think I've even asked Wendy for the name of a good bookstore in London.
While I don't consider any of these people official beta readers, still I am so very grateful for their help and thankful that we have a community so willing to share their collective knowledge.
Lynn
