Guide to winning Arathi Basin

Rule #1. Everyone on offence is a bad idea, Defend, Defend, Defend, and Defend. If you don't defend the points you capture, you will lose. Make sure SOMEONE stays behind to Defend the flag, or at least to scout/notify everyone that there is Incoming at that point.

Rule #2. Don't fight the Zerg. If you see 10 enemies in front of you, chances are there are fewer of them somewhere else. Go around the Zerg and capture points with weaker defence.
Rule #2b. Don't become the Zerg. If you look at your map and you are one of 10 or so dots at one point, leave. If too many people are at one place, that means other places aren't being defended or assaulted.

Rule #3. Do the role you were meant to do. Healers, find a DPS class, communicate and coordinate with that DPS class, follow him around, buff him and heal him. A great deal of losing battles are due to the Healer classes not doing their job. DPS classes, your JOB is to protect the healer. When you find a healer, assure him you are going to watch out for him and keep enemies off of him as much as possible.

Rule #4. Get your target priority straight. Target Healers first, this means attack and kill healer type classes first, and then DPS classes. If you attack a warrior with a priest behind him, you have already lost. Even though a healer type may not die as quick as a DPS type, at least that healer will be forced to heal himself rather than someone else.
Rule #4b. Do your best to crowd control DPS classes via Sheep, Poly, Sap, Fear, Seduce, Root, etc, to make killing the healer easier.

Rule #5. TAP! Tap tap tap tap. Tapping the flag is what captures points! Even in battle, you can click the flag and sometimes no one will notice you. If you don't tap a captured point, and continue fighting the enemy, they will shortly respawn and you will have to fight them again, capturing this point ensures they will not respawn there.
Rule #5b. Interrupt the enemy tapping. This means if you see someone trying to battle tap, stop whatever you are doing and hit that person to interrupt their clicking of the flag. If you have to, bring the fight to the flag, this will ensure you better view of the flag to make sure no one taps it.
Rule #5c. Don't fall for the bait tap. Often the enemy will draw you away from the flag only to have their friend tap it behind you.

Rule #6. Use the map! The map is your friend (M is the default map key). Watching the map will help you see which points need reinforcement and which points need defending. A useful tip is you can right click the red icon on your minimap to bring up a moveable minimap, so that you can quickly see the contest state of every point.  If you need more in-depth coverage of battle grounds and PvP tactics, check out  Spugnort's PVP guide, there's a small fee for it, but it's well worth it in my opinion.