Meet people properly
it all starts with the introduction. Get to know what people's lives are like, what kind of poetry they like, and what kind of feedback they're looking for. These are your future friends!
Find things you have in common.
You get the most out of a friendship when you know what you can talk about together, shared interests, etc. Find out if they're a huge fan of
romeo and juliet, or maybe they hate all that romantic sap. Consider posting on the forums about how your day went or what your hobbies are.
Check your ego at the door
When you discuss ideas, don't come off like you're the only one who has a valid opinion. Even when you're sure you're right, you're wrong sometimes. It's OK for other people to disagree with you.
Praise each other
Find something nice to say, even if it's a stretch. Even the worst of ideas has a silver lining inside it, if you look hard enough. Focus on the good, praise it, and then raise any objections or concerns you have with the rest of it.
Be open and honest
Talk with your fellow poets if there's a problem, and contact a moderator if you think you need help. The whole point of the site is that it's tough to get and give feedback on poetry. If we all start by realizing that's an issue, we should be comfortable discussing problems when they arise -- after all, we're all (hopefully) mature enough to communicate! Be forgiving when people make mistakes but don't be afraid to raise the issues when they come up.
Avoid conflict at all cost
When stress occurs and tempers flare, take a short break. Clear your head, apologize, and take another stab at it. Apologize for upsetting your peers, even if you think someone else was primarily at fault; the goal is to work together, not start a legal battle over whose transgressions were worse. It takes two to have an argument, so be the peacemeaker.
Don't come off like you are always right
Instead of 'You need to change the ending', try 'I think the ending would be better if...'. You aren't always right, even if you think you are, and it pisses other people off when you don't leave any sense of 'but it's just my opinion'.