When I lose, I usually go, "Darn it" and laugh it off with my opponent. Hopefully, they laugh right along. The amount of poor sports I've seen has decreased as I've played, though that may be because I've moved from the bottom ranks to the middle ranks.
I've noticed on this topic that there are a lot of kids who throw fits after a loss. That's perfectly acceptable. They're little kids. There are some kids who can maintain emotional control, and there are some kids who can't. As people age, they learn to restrain themselves. It's pretty sad when someone looks to be in their early 20s and love rubbing in people's faces that they lose, or if you win, panic like their lives are on the line. It's just a game.
That is, a fairly expensive game. You'll get a bunch of rich and spoiled folks who are used to getting things their way, and sometimes they expect to plow right through using the power of cold hard cash. You find a few kids, and some people older than kids, who haven't taken many losses throughout their lives and won't take a loss very well, or worse, the nouveau riche who like to brag about their superiority. (The nouveau riche are more likely to buy luxuries like Pokémon cards anyway, since by definition they just got a monetary windfall and are thus more apt to spend and spend.)
I did find someone at the Pasadena Regionals a little while ago who refused to talk to me. I always start with a "Hello." Of the 7 rounds I played, he was the only one who didn't greet me back. He didn't say a word until he beat me with his Palkia Lv. X deck. He said to me, "I've been losing too much." That phrase tells me a lot more about him, namely that he's taking this way too seriously, considering that at that point he had 2 total losses. He seems to have been in this for business purposes, an attempt to sweep everyone clean. Egomaniacs like him deserve to lose until they humble up and act nicely to people, not to mention consider that they're competing with three digits' worth of other people for that same prize.
