Rejoining the Battle

The attacking players have a disadvantage in their graveyard placement. They have to revive at graveyards that are much further away from the town than the Halaa defenders. Some of the wyvern posts are much further away from the graveyard than others. The southernmost wyvern post is closest to the graveyard. Keep that in mind when both attacking and defending that post. Dead players can quickly revive nearby and jump back into battle.
The defending players' graveyard is located right inside Halaa, thus defenders can quickly move to defend any part of the town once reviving.
It can be somewhat challenging figuring out how to retrieve your body if you are new to this area. Look for the paths leading up to the city from its base.
If you die in town, look for an out-of-view spot where you can revive without being immediately noticed by the enemy.
As soon as you revive, you can beat a hasty exit by escaping on your flying mount. Then you can fly to one of the flying islands, heal up, and rejoin the battle.
Defending Halaa
The side who controls Halaa receives several benefits when defending it from subsequent attack:

The powerful NPC guards will help you defend the city against ground attackers.
All friendly players receive a +5% bonus to damage for controlling the town.
The time it takes to defeat the guards gives defenders a chance to hunt down and kill the enemy.
It is generally easier to get people from your faction to help defend the town than it is to get them help attack it.
Keep an eye on the map for wyvern posts that have been taken over by the enemy. When you notice one, attack and destroy it. If any enemy players are out on bombing runs you can wait at the destroyed wyvern post for them to come back, then attack them. However, you must be at the wyvern post from where they originally departed to catch them when they return. If the enemy doesn't show up, try another wyvern post. If you watch the skies, you can see the enemy player coming in for a landing.

If you die while defending in Halaa, take advantage of the in-town graveyard to quickly rejoin the battle.

If all your guards have been defeated and the town control begins to change hands, keep attack them near the middle of town. The more players from your side you can muster in the town, the longer it takes for the town to fall to the other side.

Miscellaneous Tips
If you're flying over an enemy-held Halaa, stay way from the middle flagpole. Defenders can fly up there, land on the flagpole, and attack you while you're in the air.
If you want to buy a reward in Halaa but don't want to fight, you can keep checking your map every few hours until your side has control. Then you can fly over there and buy your reward. Of course helping take over the town yourself might be faster.
Flying mounts, particularly those of the epic variety, give you a big advantage while fighting. They are helpful for quickly reaching your allies, avoiding enemy contact, and chasing people down. They also help when you need get away and heal up.
The Arena Battlemaster is located near the city of Halaa at the Nagrand Arena. You can queue for Arena games, and then come fight at Halaa between battles.
To receive a Halaa Battle Token you need to kill a player in or near town or near the wyvern posts. Once you're out of the vicinity of the town you'll no longer receive tokens for killing players.
You can keep getting tokens for killing the same players over and over. However, the honor received from killing the same player repeatedly is still subject to diminishing returns.
Make sure your PvP equipment is on while fighting. Gear with resilience and stamina are well-suited for fighting other players.
Stock up on healing potions and other consumables before fighting to last even longer in battle.
There are certain spots in Halaa where you can fight enemy players without the guards noticing you and joining in.