1. Don't say, "I don't like this." Your job, as critiquer extraordinaire, is to comment on the craft of the piece. Chances are, you haven't enjoyed every book you've read, and the books you hated are probably someone else's favorites. You aren't going to enjoy every piece that comes through your group. Ignore the fact that you hate stories about horses and focus on how well the words flow. Does the writer 'tell' too much? Would the story benefit from a different point of view? Is "Fluffy Bunnypie" not a good name for a horse? Try to help the writer make his/her story the best it can be...even if you never want to read it again.
2. Be specific. I had an agent once who's revision request included 'making the main character stronger', and that 'the dialogue seemed forced in some places'. I stared at that letter for days. Whose dialogue? Which places? Forced in what way? And...make the M.C. stronger how? Stronger in voice? Stronger emotionally? Strong enough to lift tall buildings in a single...oh, wait. I messed that up didn't I? Anyway, the point remains. The advice was so vague it was useless. If you're reviewing a piece in dialect and the phoenetic spelling is tripping you up, don't say, "The voice isn't working." Say, "The dialect makes this very hard for me to read." If the writer gets the same comment from everyone, s/he will know exactly what to work on.
3. Be kind. Just because someone has written something truly dreadful does not mean they deserve to be drawn and quartered. It's not a crime to write bad first drafts. That's the whole reason for critiques. Try to state whatever criticism you have in the gentlest terms possible. You're a writer, after all, a master of words. You can figure out a nice way to say, "This is dryer than Mr. Norell's undergarments."
4. It's the opposite of ironing. When you iron a shirt, you start with the collar, then the cuffs. You work your way from small parts to large, so the front and back are the last parts ironed and don't get mussed up. It's the opposite with writing. If you're critiquing a piece by a fairly new author, you may crit something with LOTS of stuff wrong. If you mark every single thing (comma placement, adverbs, apostrophe's, word overuse, etc), you'll hand back something covered in red and most likely demoralize the writer. Instead, focus on the things that will change the story the most: is the plot too advanced (or too simple) for the targeted age? should the character be a boy instead of a girl? would the book be better set in contemporary times instead of the dynasty of Cleopatra? Once the big elements are fixed, you can always go back and rearrange the apostrophes.
5. Balance. Yes, you're offering suggestions for improvements, but don't forget to comment on the postive too. It's just as important for us writers to know what we've done right. If you're a person who tends to focus on the negative, orgainze your comments like this. Begin with something positive. Then, mention a couple ways you think the manuscript might be improved. And always, always finish with something positive. Think of it as a criticism sandwich...with really nice bread.
And remember, a good critique is a true gift. The more you give, the more you'll get back on your own pieces.