So, I often beta-read for my wife, who writes a lot of various sorts of fiction (1). I'm not always willing to do it (yes, your novel is important to me, but it is less important than winning this game of Huttball right now), but she usually finds it useful (2). I also do it for friends and for people on the internet who want it. So I've kind of got a few pointers:
1) Be clear about what you want it to be read for, at what stage of the writing process you are. SPAG (3) is useless if you're still trying to pull together the shape of the whole thing. If you have specific questions (Does this character work?) then it's always good to ask those.
2) Be clear about what kind of response you want back. Do you want a full edit, with suggested corrections? Or do you want a more general "this worked/didn't work for me because (brief description)" to let you do the fixing. Once again, it's about control and where you are in the sausage-factory process.
3) Be aware of the reader's prejudices and tendencies (and as a reader, be aware of yours). I, for example, jump on PoV problems like a cat with cheese, but will tend to pass over problems in characterisation that others might object to.
(1) Unpublished, but she's won proper judged competitions and stuff.
(2) Actually, it's the plot help that I find painful to give, because our approaches to writing differ so significantly that I struggle to mesh with her way of working. (She's an Explorer ("I start from here, where's next to go?"), I'm a "spot the islands, build the causeways" sort of person.)
(3) Spelling, Punctuation, and Grammar.
) is very important. A writer's going to get criticism and flaws pointed out. The whole reason for beta reading is so that you get as many flaws removed so that when reviews are written people are left with only marvellous things to say.
