I do a lot of critiquing during the busy work season. It's how I stay connected to writing. It keeps me sharp and makes me happy. I've been formulating a list of "rules" for being a good critiquer. Here's what I have so far:
Explain what you mean in a critique.
Say what you like as well as what you think could be improved.
You don't have to like the genre or even the story to help make it better.
What can you add to this list?