Basics: The objective here is to hold as many bases as possible (out of five total), for as long as possible. There are 15 people on each side. That said, you only need to hold three bases to win, so it's generally safer to grab the first three you can and just defend them. However, as other posters have pointed out, you should be aware that you can lose bases, and be prepared to react to that. If you lose one, don't spend the rest of the game trying to take that one back if there are other, less defended options.
All interactions with the flags are channeled, and get interrupted by any "active" damage(dots/passive AoEs like DnD don't). When you tag a flag, you have to wait a minute before you start gaining resources. Also, a flag that's in conflict has no active graveyard. If you die there, you'll be set elsewhere to rez.
As Always: COMMUNICATE! It doesn't matter if you're trying to take a flag or defend one, make sure you keep your team in the loop. If you need help, let them know. If you see a base empty, yell it out. And come up with a plan to start with. It doesn't have to last long, but you don't want to lose a match because no-one tagged your closest base (trust me on this one).
Defense: You always want at least 2 people at a node you're defending. This protects against one person ninjaing a node by CCing you and grabbing it while you watch (for example, a rogue sapping you, or a ele shaman knocking you off LM). It's also generally advisable to try to keep one healer at each node if you can, since it'll allow you to hold out for reinforcements longer. That said, offense will generally attack one node in strength at a time, so healers should be ready to move if need be, especially if there aren't many on your side.
For defending the flag, remember that killing people is secondary to protecting the flag. It's important to kill them off (since that means they're no longer a threat), but it's more important to stop a cap. If you see someone trying to take a flag you're defending switch to them immediately and hit them with something direct. It doesn't have to be powerful. For example, holy nova. Furthermore, if you do get killed, you can move your ghost a few steps closer to the flag. Don't go too far (since you'll leave the rez area), but a couple extra steps can mean the difference between barely stopping a tag in time, and losing the base. Also, don't give up if they do manage to tag your flag. You still have a chance to kill them and take it back, or to ninja it back while they're distracted.
Finally, if you see the other team leave a base empty to attack one of yours, it can be worth it to go tag that flag. It can work as a great distraction to force them to quit the attack, or it can serve as a replacement if you end up losing the one they attacked. This is a bit risky, so I wouldn't recommend it to people who aren't that experienced at AB.
Offense: First off, never fully abandon a base. If your team is going after a third base, and you're the last one left, don't leave. If you lose a base you already have in order to gain another one, then it's not that much of a gain. Secondly, remember that the objective is the flag, not the defenders. You always want to look for the least defended node, and attack there. If there aren't many by the time you get there, kill them asap and tag the flag. If it'll take time, try to kite/CC the defenders and tag the flag. Even if you can't get them all CCed or out of range, you can sometimes grab a flag in the middle of a fight. You don't want to just sit there spamming it while you're being attacked, but if you notice you're not being attacked and are close to the flag, it's often worth it to try once. After you tag a flag, you can then work on killing off the defenders. The longer the fight goes on without the flag being tagged, the harder it is to take a base, since the defenders have more time to react to the attack, and have a graveyard that's close by. Once the flag is successfully assulted, then you can focus on killing the remaining defenders. Be aware that you should keep an eye on the flag, since they'll likely try to ninja it out from under you.
Another thing, if you notice that a base is empty, it can often be worth it to try to take with one or two people. That'll allow you a chance to take it without draining the rest of your defense/offense, and will be less likely to be noticed by the other team. This is especially true for rogues and druids, since you can stealth into a base, and CC the defender if there's only a few there.
Faction Note: Horde will generally get to the BS flag faster than alliance due to the map design. If you're an Alliance DK or a Shaman, be sure to throw up your appropriate water walking ability. If possible, get a paladin with crusader aura to come with you (especially shamans). This can get you enough of a movement advantage to get to the BS flag in time to prevent the first tag by Horde. You can then leverage this into a simple delaying tactic (if you're not in force) or a full take over (if you are). While delaying them a little may seem pointless, remember that bases can go back and forth. If it's a close game, then an extra 30 seconds longer for them to take BS could be all it takes for you to eke out a victory.
Edited by Karat on 3/26/2012 9:35 AM PDT