Two flags, get the enemy one, stay alive and have your friends kill the enemy flag carrier.
This is a BG where a tank shines.
I've learned to dispense with all finesse. Just ride across the field, keep a little bit to the right to avoid getting caught up by enemy CC. Ride straight through the front door and keep running.
This can be done even without escort, but if the flag is guarded by more than two people you're dead. Preferably you have a squishy with you to grab enemy attention.
Get the flag, and avoid getting involved in any fights unless you're zerging the place. Run back out the same way you came. Don't forget to check for the speed-buff.
What I've found working out best is to cross the field somewhat on the outskirts of where fighting is going on. You're likely to draw attention, but unless the team you're grouped with are hopelessly bad you'll also get yourself a pretty strong escort.
The reason getting slightly caught up in battle mid-field is actually pretty good is that you'll draw fire onto yourself. Your healers are likely to be left alone, and with a live healer you can crawl all the way to the flag-deposit point without going down. I even go as far as backpedalling the last bit to make sure enemy players don't lose interest in me. By that time I've stacked up an impressive amount of vengeance, and reds quickly learn that a flag carrier actually hurts pretty bad.
The kind of attrition fight described above should normally give your dps a decent opportunity to catch the enemy flag carrier in superior numbers. The reason for this is that while you slowly fight your way home, the closer to your own spawn point you get. As players die left and right you'll generate a continuous superiority in numbers simply because the enemy has longer to ride to get back in action.
Back at home you just cram yourself into a corner by the deposit point. Hopefully you have a small escort including a healer staying with you.
I know a lot of people prefer staying upstairs, but my experience is that this is all too often a very bad idea. If an enemy DK pops into the fight you're likely to be physically separated from your healer. It's a lot harder to split up the defending group if they're close to the flag.
When the enemy arrive, and they probably will, you'll need to check their numbers. If they're few you most likely want to leave your protected position and assault in order to help with local numerical superiority. Use SoT and let them eat shield.
If they're a crap-ton, make sure you're seen and back into that corner again. Hopefully one or two enemy melee dps beeline directly for you, giving you a superb opportunity to tank them PvE boss-style. Even though some 25% block chance is rather poor, it's a lot better than zero. Add that your measly dodge and parry both activate if you're fighting frontally. It all adds up. Your stun is a defensive cooldown as well. Use it.
This is also the point when you need to decide if you're going to bubble before going down or if you're going to risk being CC when you need to pull the LoH off. Always remember that bubbling makes you drop the flag, so this has to be timed. As for the general fight just distribute your defensive cooldowns evenly and keep a very close eye on your healer health. You'll want to bubble him/her when things look dark.
Also don't forget your raidwall (if you're specced into it) and the measly area heal you have available. You should probably fight with SoI rather than SoT when overwhelmed numerically as well. WoG at CD, so use that SotR sparingly. If you have time and are able to see anything, free friendly melee dps from slows.
Usually nothing funny will happen until the flag carriers have collected four stacks or more of the damage debuff. This is also when enemy zergs usually occur. Eat food, bandage and throw self-heals between attack waves. Also, don't forget to buff up occasionally.
When the flag has been captured you need to know if you're one or two flagcarriers in your team. Sometimes a squishy grabs the enemy flag. In that case you need to get out to orchestrate a drop off. If you have a proper tank picking the flag up, then you should join the offensive team. Not because you're especially good at killing anything, but because you want to be close to the enemy flag when it spawns after the next capture.
Now get out there and tank your way to glory.