Frank Fleming put out a little post on how NOT to appear crazy on the internet. It is a helpful piece. Let me summarize some of his points here, and perhaps add to them...
1. Avoid the caps lock button. The use of all caps is like shouting to those next to you. It is as crazy on line as it is screaming in a discussion.
2. A corollary to this is to watch and limit your use of the exclamation point.
3. Watch your grammar and write in actual sentences, using real words (as opposed to made up ones). Do not write as you would txt.
4. Do not write tomes; limit yourself to short, clear, and cohesive writing.
5. Proofread what you have written before you hit post, send, or comment.
6. Don't be surprise if folks disagree with you.
7. Hitler is dead. He has not come back in the form of your critics. Resist the great temptation to demonize your critics.
8. Respond to actual points the author has made and not your misreading of their opinion or some straw man you raise up only to tear down.
9. Do not make your comments into a running battle against the original author or another commentator. Know when to leave well enough alone and move on.
10. Get a life. Do not live vicariously through the internet but take some time off to really live.
11. Do not take everything personally and do not make everything personal.
12. If the owner of the blog or online forum deletes your post or comment, remember that he or she owns the domain and gets to make the rules. If you don't like them, don't go back. Sometimes the internet is like an addiction and every now and then you need to go on the wagon just to maintain your sanity.