There's so much that makes a good player, many of the ways I've seen through my pals:
Behavior:
1. Like school or work, come prepared. COME with dice, COME with 60-cards, all sleeved, and COME with a good attitude!
2. Respect everybody: yourself, your opponent, and the judges. Most prepared sanctioned tournies carry a copy of Compendium ex, so if there's a debate, then no need for a fit: try to figure it out on your own, but go to that if all possible. It's even handy for the player him/herself to have one around.
3. On the flipside, if you're absolutely sure you're correct on something, don't be afraid to debate with judges or opponents, as they don't know everything ('though I can name a guy or two who DO
)
Deckbuilding:
1. Don't blatantly steal a deck off the internet and expect to win. It's not bad at all to play the same archetype, just edit it as you see fit. In writing, there's plaguerism, and its counterpart, paraphrasing. In TCG's, there's stealing ideas, or merely teching out for local metagame.
2. Open minds count big-time. Don't knock off a card just because it doesn't seem as good as 'Quaza or Garde. A shining example would be about any Pokemon from the original E-sets. Dudes like Ampharos (exp), and Ninetails (Exp) were played by very few until Tyranitar666 and Venusaur revived the old stuff, and made pretty solid decks out of 'em (I'm referring to Blaziken/'Tails, and BAR). Another notable mention would be ToysRUsKid and Super Saiyan Eevee and the E-on version of Donphan/Baby, using Gengar/Wobs/Shed. Of course these aren't the only ones: new stuff's going on everyday, and each and every one of us has new ideas that could annihilate the competition.
Philosophy:
1. Be your opponent's friend and enemy at the same time. Make very brief, idle chitchat before, during, and after the game. However, have a strong-enough character to keep your spirits up, no matter how bad things get. Thanks to Nintendo and a lack of any cuts to T8, City Championships will only warrant a VIP package to a person who's undefeated. Since there's no chance if you lose even once (excluding the select few places who may be running a T8...I have no idea =/), it's vital to do what you have to...just keep a clear head!
2. Tying into #1, avoid any and all stupid, careless errors. No doubt I've made some, and no doubt all of you have, as well. These errors cost us games, and even tournaments. Don't lose $100 worth of product just because of a simple mistake.
3. Be a good sport. You're there to win, but you're there to have fun as well. Don't treat your opponent like crud, because you WILL get kicked out for unsportsmanlike conduct. Again, another careless error like THIS can lead to lost oppurtunity, probation, and even bans. Fun stuff, eh?
There's a lot that goes into the actual gameplay, but you see that everyday on the boards, so I won't re-state any of that. Good kids get gifts from Santa! =P
~patriarch