When it comes to critiquing is is truly better to give than to receive. Receiving critiques is important, but generally, while you may get advice that can carry into your other work, getting a critique will only help you with that story.
Giving a critique, on the other hand, opens your eyes to what works and doesn't work in a story like nothing else can. Critiquing forces you to define what engages you in a story and what throws you out. What makes the writing invisible and what makes it intrude.
One of the best things I ever did for my own writing was to become an editor for Mindflights. I have learned so much about what it takes to write well just from reading and judging submissions. (If you ever get a chance to do something like that, take it!)
So, a few rules for giving good, useful critiques:
#1: Be honest! This is the first and foremost rule in my mind. A critique is useless if you aren't truthful.
#2: Be kind. Yes, you can be both honest and kind. Remember the feelings of the person receiving the critique. You have to state your honest opinion, but you can do it in a tactful way. (You can be blunt, but don't be rude.)
#3: Remember that it is just your opinion. The other person does not have to take all of your suggestions and that's okay. I usually prefer not to know if they've taken my suggestions. (Unless I've asked for a rewrite on a submission. Then they'd better take my suggestions!)
I should probably state for the record that I don't belong to a critique group. I have not found them effective for me. Luckily, I have some trusted writer friends who are willing to give me feedback. Maybe face to face critique groups have different rules to consider. I don't know.
What has been your experience with giving critiques?