Disclaimers and Caveats
"I don't know if I'm right, but..."
"...Or I could be totally off-base here."
"Well, I'm not sure if I read this correctly, but..."
"Just tell me if I'm wrong, but..."
I'm saying this as someone who has been there, as well as someone who was horribly, hideously shy as an undergraduate - stop putting disclaimers and caveats on your statements.
It is particularly damaging when you get to graduate school, and many classes are dialogue-based. Students are expected to critique and be critiqued, while offering original thoughts on the course material.
I remember the moment I realized that I was undercutting my arguments before I even made them. I was an MA student at Harvard, in a tiny seminar with History PhD students. There was only one other female student. My male classmates were a little older. Everyone was (or at least appeared to be) way more confident than I about their interpretations of the texts. After weeks of observation, I finally (finally!) realized what was going on:
The students who made their arguments, without any disclaimers about possibly being wrong, were the ones who I was most inclined to a) listen to and b) find persuasive.
Ohhhhh. You mean when you say something without apologizing for it upfront it comes off as more authoritative? (Blergh. Why did it take me YEARS to have this non-revelation?)
Fortunately, there is an incredibly simple way to rectify how you deliver your arguments to your classmates and professors: Say what you're going to say, with zero disclaimers. Let others determine if they want to question the validity of your statement (and there's always somebody who will). Then respond to their point by either defending your argument, or by conceding you find their own position more tenable.
When Receiving Critiques
That is a really great point, and here's where I think we are agreeing about...
That is a really great point, and here's where I think we are approaching the topic differently because...
I appreciate your feedback, and I'm not sure I completely understand. Would you mind explaining a little bit more?
I'm going to need to take a minute to think over what you said. Can we come back to me?
Hmm. Actually, I'm thinking about what you said, and I have to say, I agree with you. Thank you for pointing that out to me.
However, if someone is just being a real jerkface when critiquing you, feel free to devote less time and energy to defending yourself. If someone makes a personal attack on your character because she doesn't like your interpretation of The Great Gatsby, then, um, just let her look ridiculous in front of everyone else and try not to get sucked into a stupid argument.
When Critiquing Others
What I mean by that is don't confuse debating about a concept with being in an angry argument with your classmates.
I've seen this happen and it's too absurd when someone throws a tantrum over the relative importance of theory in History courses.
So, in addition to the obvious rules of don't raise your voice, no name-calling, and make sure you have a valid critique, try wording criticism in the same way you would want to receive it:
I understand what you're saying, and I'd like to add that...
I hear you, and think we could also consider...
I think you're argument makes sense, and I'm coming at differently...
While I agree with your first point, I was also thinking...
critiques happen. deal with it.
Stop undermining your arguments with caveats or disclaimers. If you don't believe in your own argument, why should anyone else?
And make sure to offer critiques in the same language and manner you'd want to receive them. The surest way to undercut your argument is by pitching an angry fit every time somebody disagrees with you.