Pet peeve of the day
Here's a good rule of thumb: don't declare yourself the winner of debates in which you are a participant. Nothing smacks of more arrogance or narrow-mindedness than making a point of mocking the weakness your interlocutor's arguments or triumphantly announcing the decisiveness of your own instead of simply making your point and letting your arguments stand on their own.
Interestingly, some recent research has indicated that people who use declarative language are much less receptive to contrary points of view anyway. In other words, people who feel the need to speak as such are likely so convinced of their own opinions that they're almost certainly not going to change their minds, no matter how powerful the evidence and argumentation to the contrary.