Teamplay is the primary point of CTF. Because of this, it only takes a few (ahem) misinformed players to ruin the game for their teammates. Don't be a bonehead... learn what it takes to become a valuable teammate and a respected foe. Here's a few things you should keep in mind:
Things not to do
Don't attack your teammates: This seems totally obvious, but you'll be surprised at how often it crops up. Since your teammates don't take damage from your attacks, it's not a big deal if you accidentally fire off one or two shots. In fact, shooting a teammate is probably preferable to not shooting an enemy, so if you're unsure by all means fire away. However, you will occasionally run into a player that attacks his teammates all the time for no reason. Perhaps he is unaware that he is on a teamplay-oriented server, or perhaps it's just his way of being friendly. Either way, it is both extremely distracting and a waste of ammo. Don't do it.
Don't pin your teammates: Picture this scenario: you're standing in a shadowy corner of your base, rocket launcher in hand, ready to make a nasty mess out of anyone wearing the wrong colors. Suddenly one of your teammates walks right up to you as if to take a good look, and just stands there staring at you and making it utterly impossible for you to move. Of course, just then you see some enemies racing into the base. This scenario usually ends with both you and your curious teammate dead. The moral: don't do that!
Don't cross-dress: The CTF server modifications can lock the pants color, but not the shirt colors. Quite often dishonorable players change their shirt colors to match the enemy's, attempting to confuse them. It works to some extent, but it also confuses your own team and is considered rude by most CTF players. Note that this problem does not exist to the same extent on Custom servers, where players sport stylish new uniforms with solid-colored bodies. So if you crossdress, you will now often simply make people angry without gaining any tactical advantage at all. Not a good idea.
Don't be greedy: If you're stocked on weapons and ammo, don't automatically pick up every goody that respawns in your base. While you're sitting pretty with a rocket launcher, perferator and double shotgun, the teammate next to you may be down to his trusty but wimpy single shotgun. The same goes for life and armor: don't pick it up unless you really need it, because someone else probably needs it more. Note that while you should give teammates a chance to pick up weapons, you probably shouldn't leave heavy weaponry just lying around the base for invaders to grab. If no one takes a weapon after a reasonable time, picking it up is probably a good idea.
Don't take what isn't yours: Don't you hate it when you've finally managed to kill that strength-rune-toting enemy and a teammate swoops down and scoops up the rune before you can claim it? Or when you've killed the enemy flag bearer and are just about to recover the flag when a teammate runs in front of you for the extra point? Or when you've disposed of a rocket-launching foe bare-handed but your teammate claims the weapon for himself? It's not polite to take a prize that belongs to someone else... don't do it if it can be avoided. On the other hand, don't be too quick to condemn someone that does take something you earned... it's easy to accidentally grab something you meant to avoid.
Don't be an idiot: This is a pretty wide-ranging don't, but it is necessary to cover a wide range of unsavory behaviors. Not being an idiot means not accusing base guards of "camping". It means not accusing people of cheating simply because they're good players. It means not expressing your personal frustration by cussing out other players for no particular reason. Simple heuristic: if what you're about to say is rude, pointless, and annoying... don't say it. Like any game, CTF is a lot more fun without jerks--and a lot more fun for people who aren't jerks.
Things to do
Be a team player: The only way to win at this game is to operate as a team. The temptation is to try to be the hero every time and make a capture yourself. But keep in mind that capturing is impossible without a solid team behind you. Know your job on the team and stick with it -- be dependable, especially if you are playing defense. If you get back from a capture run and find your base empty, take a turn guarding it. Someone's gotta do it. Sacrifice yourself if necessary for the team: it is just as heroic to take yourself out along with the enemy flag carrier as it is to capture the flag yourself. Check out this page for a description of some typical CTF team positions.
Come to the rescue: If you see a teammate getting attacked, chances are he'd appreciate your assistance (especially if he's been grappled!). Even if you've only got a grappling hook, your help will make the battle much more difficult for the enemy. And remember: if you watch your buddy's back, he'll be that much more likely to watch yours. Keep in mind, though, that if you're in pursuit of the enemy flag bearer or carrying the enemy flag back to base, those missions take priority over everything else.
Communicate: This is important enough that we have devoted an entire page to the subject.