Never dismiss a feature just because you don't see a use for it. Other people might use it all the time. Examples: MultipleInheritance, Closures, ...
Never state that a construct seems preferable without explaining why it seems preferable.
Use EprimeLanguage. That's to say, try to stop yourself from saying things like "ComIsLove". Explain instead what seems lovely to you.
Another useful technique is to expose one's lack of understanding. Like "I don't understand how one implements polymorphism in VisualBasic", or "I don't see how X works better than Y." Others will be glad to enlighten you. (Be sure to avoid BrutalSarcasm, however--it rarely works in writing.)
Try to avoid 'league tables'. (e.g. 'More shrink wrap apps are written in Language A than Language B'.)
If you are debating a non-technical issue (e.g. extent of penetration into a market) try to remember that it may have nothing to do with technical merit.
Question your motives. What do you want out of the discussion, do you want to learn? win? comfort yourself that you will still have a job in 5 years? decide which language to go for next in order to still be in a job in 5 years?
Do you feel angry when you read criticisms of your language? Do you feel undermined? Better work out why; these things are just not that important.
If you are debating to score points make it clear up front and put Advocacy in the page title!