1. Survivability. The first rule of AvA is survivability. With the exception of highly stealthy builds that consume a LOT of potions, glass cannons do not last long in Cyrodiil. With the removal, for now, of forward camps, suvivability is all the more important, as you will either need to wait for someone to resurrect you or ride back several minutes to the battlefield when you die. There are four contributing factors to survivability in AvA; healing; high or boosted defense; evasion; and one of the most overlooked - constantly moving to make yourself hard to target. I'll deal with these each in order below:
A. Healing. Everyone one wants to damage deal or tank, but healing is absolutely critical. If you want to be part of a group that rolls around the battlefield surviving enemy assaults, at least 30% of your group should be dedicated to healing. So if you have a group of 10, at least 3 people in that group should be primarily casting heals in combat; if you've got 20, then 6 group members should be dedicated to healing primarily. If you don't have many dedicated healers, you can use hybrid healers (folks who partially dd or tank and partially heal). But if you do, I suggest doubling the number. So if you have a group of 10 and no dedicated healers, then 6 group members should be templars or other classes with resto staff as their off-hand weapon.
There are two types of group healing and you need both: burst heals and HOT (heal over time) heals.
Resto Staff: Rapid Regeneration/Mutagen, Healing Springs/Illustrious Healing,
Templar: Purifying Ritual/Extended Ritual,
Nightblade: Funnel Health.
Resto Staff: Combat Prayer/Blessing of Restoration
Templar: Honor the Dead/Breath of Life (the best AvA heal in game, imo), Ritual of Rebirth/Lingering Ritual, Repentance, Practiced Incantation/Remembrance
Nightblade: Sap Essence
Additionally there are healing related buffs that do things like increasing health regeneration or increase the healing received:
Templar: Radiant Aura, Restoring Focus,
Fighter's Guild: Ring of Preservation,
Undaunted: Sanguine Altar/Overflowing Altar.
The most common heals in AvA are Rapid Regeneration, Healing Springs, Breath of Life, and Sap Essence, but each of the above are useful. As a healer, you want to keep up both the HOT's and use the burst heals when the health bars start to really go down. There is a dance between managing your magicka pool and keeping up the party health that you can only acquire through experience - just get out there and find your rhythm.
B. Defense Shields are your friend, as are buffs that increase Armor and Spell Resistance. It's very helpful to your group's survivability to keep several forms of the following up in combat:
Resto Staff: Combat Prayer/Blessing of Restoration, Healing Ward/Ward Ally,
Dragon Knight: Igneous Shield/Fragmented Shield,
Nightblade: Veil of Blades,
Fighter's Guild: Ring of Preservation/Turn Undead,
Undaunted: Spiked Bone Shield/Bone Surge,
Alliance War Support: Reviving Barrier/Replenishing Barrier.
Additionally, there are individual shields (or in the case of a sorc, specific to them and their pets) that decrease the need for healing (Templar's Blazing Shield, anyone?).
C. Evasion. Evasion did not seem to receive a lot of attention from the AvA crowd until Stamina builds got buffed in the fall. Evasion comes from two primary sources: skills, and dodging.
Medium Armor: Elude/Shuffle (Elude is the better morph for evasion
Nightblade: Mirage/Double Take
Templar: Searing Light/Blinding Flashes (this skill and morphs are about to be replaced by a ranged execute for templars)
Dual Wield: Heated Blades/Ember Explosion
D. Stay Mobile. Moving around helps alot in both 1v1 and large group combat. In 1v1 or small group combat, keep moving around and particularly keep moving behind your target making it hard for him to keep you targeted, hard for templars to stun-lock you with their spears, etc. In large group combat, move up to attack range and quickly move back, move laterally along the line of you allies keeps yourself from getting mass targetted by a bunch of enemies.
A. Timing of a seige. Sometimes you just need to force the issue and start a seige when nothing else is occurring on the map. Often, however, that is not the case. If you have a little patience, the enemy will engage either you or the third faction and a window of opportunity occurs to hit them in another location while their forces are engaged at their original seige. Too many times people zerg to the next nearest keep. If the enemy is pushing the majority of their forces to one or two areas, there are likely unguarded keeps behind them. Prep both the inner and outer walls (hit the inners with trebuchets while the outers are still up) to around 52% before you flag the keep, so that when it does flags, there is a greater chance of your raid getting on the flags before any defenders can reach the keep.
Another fun thing to do, which we've done once or twice in Blood and Steel AvA, is to seige multiple enemy keeps when they are pushing your faction aggressively. One night DC kept pushing Chalman and Kingscrest hard. So we went all the way west, deep in DC territory, and flagged all three of their scroll keeps at once - Rayles, Warden, and Glademist. Though we were a small 9 person raid, we almost took Warden and Glademist and DC lost Chalman, Bleakers, and Aleswell in their panicked response to our triple-seige of their scroll keeps. DC was gun-shy for the rest of the night. They re-took Aleswell eventually, but we did not see the aggressiveness they had previously showed at KC and Chalman.
B. Timing of engaging an enemy group. Think about what you would do in an enemy's situation on the battlefield. If they are charging a choke point or friendly zerg, or if they are charging a flag, they are likely going to spend their ultimates and a lot of their resources in the first 5-8 seconds. If the situation allows it, hit them, ranged or mass charge, after they have exhausted those ultimates and resource pools. Also, think about when enemies are least likely to be watching their back. Hitting the back side of a seiging group as they charge into the inner keep can sometimes cut them in half and you can kill 1/3 to 1/2 of the group before the others realize what is going on and they are then engaged with the guards.
C. Timing of resources. This one is tricker and takes experience. When you get engaged in a prolonged 1v1 (I've had some last more than 10 minutes), who ultimately wins usually comes down to who manages their resources (mana/stamina/health) best. it's not just about the health bar. Against most enemies, it is essential to try to strategically drain your enemy's stamina pool and then crowd-control (cc) or stun-lock them when you think that stamina pool is low. If you can cc or stun-lock you enemy for 3 seconds and they dont' have stamina to break it, then the duel suddenly turns into an easy kill. how do you get them to drain their stamina? Make them block or dodge roll or in the case of a stamina-build opponent, heal through their weapon skill attacks and when it looks like they've spammed a lot at once, try to cc/stun lock them. Again, this is something you develop a feel for with experience.
3. Stay together. Good, AP generating groups know how to stay together. They ride together and in the midst of combat they stay close on the Crown except for when the group leader yells for party members to disperse. I've grouped up with several very successful raids in the past month and I've seen them heavily use survivability and staying together to great success, often destroying groups up to 4 times their size in an outpost/keep defense and up to twice their size in the field. Staying on the crown, keeping moving, keeping the heals and shields up can yield amazing results.