The towers cap over time depending on who has the most people standing on the site. You want to position your people so as to push the enemy off the site. Don't let the other side kite you away unless you are SURE that someone is staying to defend.
Put ranged near the crest of the hill, shooting down.
Melee should try to drag the enemy away from the tower.
The goal is to create a melee in the direction of the gy ... far enough from the tower so that their people aren't contributing to a cap. The closer you are to the gy, the faster your people get back into the fight.
It is worth noting that the Honorable Defender buff that you get while guarding a tower increases your honor gain by 50%.
Guarding Towers
Towers are where points come from ... so focus on keeping them. Always leave a team of defenders at every tower you own.
Try to keep three guards at each tower. Never leave a tower unguarded ... and one guard is just not enough.
The optimum team is a healer, a cc and a tank/dps ... a druid healer, warlock and death knight is nice, for example. You can't always get what you want, of course, and the practical approach is just to make sure that three people are there and that they know how to defend.
Make sure the guards know they should call incoming as soon as they see the attackers coming. EotS can be hard to traverse, and support troops need time to get there.
Try to have a designated support group. It's easier to say "group 2 to MT" than to try to organize the support on the fly.
It is better to pull out of a fight for the flag in the middle than to lose a tower.
Resting
EotS is non-stop fighting for an extended period of time. People get tired. Having a tower rotation set up might be a good idea.
Work out a way to rotate the tower defense. For example, when someone rezzes, they might go to a tower and replace one of the defenders.
You may have a healer or tank that you want to keep in the fight all the time, but most players will benefit from a short stint on guard duty.
Guarding Towers