I haven't been on this site long, and I haven't even finished a throwdown yet (I am winning in all 3 of my unfinished ones, though).
But just from my observations, it seems like there's a pretty solid formula for winning them. You can really tell who realizes this and who doesn't. So here, from what I've gathered, are the ways to win a throwdown, or at least to get me to vote for you:
1.) Don't be a jerk. Nobody votes for the jerk. In about half of the throwdowns I've looked at, one of the contestants acts like a huge ****. I keep seeing things like "Are you serious?" or "This is a stupid throwdown." This doesn't work. Throwdowns are supposed to be spirited debates, not insult-offs. At least pretend to respect your opponent, or you end up looking like a buttwipe.
2.) Concede some points to your opponent. Start out arguments with things like "It's true that______, but...." or something to that extent. It kind of goes along with #1 in that it makes you look like less of a douchebag.
3.) Use a dictionary. It's really amazing to me how much more likely I am to vote for an argument that says "Although the Heat won the title last year, they are outmatched this year" than one that says "altho the heat one the titel last year, they r outmatchd this yaer." Some of the arguments I read make about as much sense as telling dead baby jokes at a pro-life convention.
4.) Use stats, or numbers to back up your argument. They make you seem smarter, and if your opponent doesn't have numbers of their own, it's harder to argue with yours.
So there it is. Probably not very helpful to anyone, but I just thought I'd put it out there.