Remember that there are real people at the other end of the wire and that they have real feelings
When someone reacts poorly, try your best not to become defensive
It's telling that the first two points here are based in negative situations. Read: People can be jerks. Try to chill.
There are sometimes a lot of conversations going on in an IRC room. If you have a conversation which involves only one other person, do the room a favor and take it to a personal message. This will reduce the amount of noise in the room.
Try to stay on topic. Sometimes a room will get a bit goofy, but in general, things usually stay on the topic of the room channel.
Never forget: Most open source project IRC channels are publicly archived. Don't say stupid things (in general, but also) which you don't want people to see later. A diamond is forever, but it still doesn't last as long as one bad comment/photo on the internet.
A lot of channels have "bots" which will auto-post to them. When you first join a channel where you plan to spend a lot of time, ask someone where you can find information about the channel bots. Using these bots can be both fun and informative.