Be familiar with the group you are posting to before you post. You shouldn't post to groups you don't read, or to groups you've only read a few articles from, you may not be familiar with the conventions and themes of the group. One normally does not join a conversation by just walking up and talking. Instead, you listen first and then join in if you have something pertinent to contribute. Without the voice inflections and body language of personal communications, it's easy for remarks meant to be funny to be misinterpreted.
Subtle humor tends to get lost.
Take steps to make sure that people realize you are trying to be funny. It points out sections of articles with humorous intent. No matter how broad the humor or satire, it is safer to remind people that you are being funny. But also be aware that frequently satire is posted without explicit indications. If an article outrages you strongly, ask yourself if it may have been unmarked satire.