This is going to be a quick post, but I felt it was something that should be gone over, however briefly. This is very similar to table etiquette, and in fact a lot of things said here may also appear on other blogs, or even this one, on posts about table etiquette. However, Good Sportsmanship is different, and it is something that is even more important than good etiquette. You can have great table manners and still be the world's biggest dick. So lets look at what you need for good sportsmanship.
This is the first and most obvious thing needed for being a good sport is to lose well. Losing sucks, we all know this, but the Good Sportsman understands that it is just a matter of life and the game. So swallow down your pride, congratulate the winner, learn from it, and come at it again. If you feel like something went down in a bogus manner, then talk to the GM about it quietly after game, not in front of everyone else at the table. Ultimately, the best way to lose well is to understand that the victory will come eventually if you work for it. It will come faster if when you lose, you lose well. Afterall, others will help you out when they see you're not going to be a pain in the ass any time things don't go your way.
Just as important, perhaps even more important, as losing well, is to win graciously. It is the law of the dice that eventually you will come out on top, and when you do that isn't the time to go rubbing it in to everyone's face. Remember when you lost how it felt? How you had to swallow your pride and bite your tongue? Yeah, that is how the others are feeling right now. Even if the other PCs didn't lose, you don't want to alienate them in your moment of victory. They'll get it, emotions run high and a bit of celebration is good and fine, but don't over do it. Win graciously, and people will even help you continue to win.
No Dick Waving
I was trying to think of a way to say that more PC, but ultimately that is what it is. Don't dick wave. Yes, you love your character, and I love my character. Comparisons are bound to happen, but when we get into the "well my character could totally cream your character" argument..well, it is annoying to everyone. It's also just a level of competitive dickery that doesn't need to be done. You really want to know how it would work out? Do a mock battle with dice and everything. Otherwise, just leave it alone and be quiet.
Final Thought for the Day
That is the three big ones for today on being a good sport. I'm sure there is more. Perhaps you have mor that you'd like to share? But ultimately, no matter the game you are playing. Lose Well, Win Graciously, and Do Not Dick Wave. Things will go much smoother for everyone that way.