First, I recommend trying to learn what you can for yourself out of the critique process. Over the early years, I learned much more about the craft from critiquing than listening to critiques. Process: you think, "well, this isn't quite right. Hmmm, I wonder why? " You think about it, try some alternative ways of doing what the author was trying to do, and finally you get an "aha!" and you've learned something about writing that will stick with you. See doing the critique as a golden opportunity to learn.
Second, there is no reason to be cruel. If you really, really hate something, say "I don't think I'm in the intended audience for this," and focus on some specific craft issue like "I notice you rely on visual images and I'd like the occasional smell or sound, too," and stop talking and let the next guy talk. If it's verbal critiquing going on, short critiques are nice (most people go on and on and on and repeat what's been said before). On a related note, I like to own my possible shortcomings. "I couldn't get into this, and that may be your fault or a concentration problem of my own last night, but here's where I drifted off." It actually might not be the text but me who is deficient. If I'm the sixth person up, and other people got some complicated thing that was going on and I hadn't and hated the thing, I admit it. "Listening to those who just spoke, I see I missed the whole point, and I'll re-read and give this back to you tomorrow. Because I didn't get it, I pass."
Orson Scott Card says there are three core negative reactions by readers: huh? (I didn't understand this), no way! (there's a verisimilitude problem) and so what? (I'm not engaged enough to care, or it's an irrelevant detail or overly long passage of description I didn't need). That's one way to think about critiquing, marking where you were confused, where you had not yet been convinced that a character would do that (or that physics works that way), and where you lacked sympathy for or belief in a character.
When you're getting your own work critiqued, write furiously. That way, if someone is being a jerk and you want to defend, blush, or weep, you can focus on writing instead and not have to lift your head if you don't want. (You don't need to write down what the nasty one is saying; you can write "this guy is a jerk. His crack whore of a mother should have used birth control." Who'll know but you?) Never defend or explain unless asked a direct question and then keep it to one or two words. Thank everyone for taking the time, even if your feelings were hurt. They've given you the gift of their time and attention, and gratitude is the correct response.
