1. Don't allow yourself to get angry or perturbed--realize that losing an argument is distinct from getting your hindquarters screwed.
2. Don't prevent yourself from getting angry merely by minimizing the evil of what your opponent is arguing for--better to be angry than to have compromised the truth.
3. If you are not angry merely as a result of a conscious resolution not to be so, and if the people you are arguing against are likely disgusting abusive types, nevertheless be true in tone and manner to the angry emotions you are trying to avoid. This will have two effects. First, the people trying to unsettle you will go totally ballistic because they will think they can win by unsettling your emotions, and they will look totally ridicuous in the process. Second, the decent people who read your post will respect you more because screwed-up people tend to be excessively laid-back.

The abolitionist Wendell Phillips (often considered the greatest orator of the 19th-century) was the master of these techniques. In one famous speech, after every time he pronounced the name of his opponent he would pause to get a glass of water, then to swish his mouth out and spit.

As for how to be convincing when arguing with people who are basically decent and reasonable, I'd say offhand that there's not much you can do except to have interesting opinions, preferably presented with well-reasoned interesting arguments.