Chewie, have you ever seen The 40 Year Old Virgin? They give Andy a bunch of advice that sounds stupid for the sake of making jokes, but for fun I tried it all out, and it gets good results. The most important one is "keep it on her, don't talk about you at all." The rationale behind this, I've concluded, is that you are a stranger. They don't give a shit about you, so you talking about yourself just makes them feel like you're not actually paying attention to them.
So, give complements, but not physical ones, and try to make them as subtle as you can. I don't know if it's just mental Judo, and you have to convince them that you think they're great without them noticing, or that they do notice but need to be able to pretend that they don't so that they don't feel conceited. Maybe it's other reasons, I can't say because I'm not a mind reader.
Don't send a single generic message. Always tailor what you write to what they have written. That't the single most important thing: it's always about them. If you can make jokes, do so, but if you can't, don't worry about it. If a girl doesn't have a lot in her profile, and there's nothing you can relate to or find something to talk about, just pass her up. If a girl isn't going to make an effort to make herself approachable by giving you things that you can talk about, don't waste your energy. I think of empty or minimal profiles as the first sign that a girl isn't actually trying. If she's not trying, you don't need to.
All of that takes practice. I think I've been using OKC for almost 2 years now, and I've only gotten to the point that I get messages and profile views regularly within the last few months. Keep track of your successes, and forget your failures. You'll learn intuitively what works for you.