There is a lot to discuss and I am glad this thread was started - maybe the reading will help those just getting in the game shorten their learning curve.
Pvp - 1v1 or small gvg is different than sieging IMHO. These fights are guerilla warfare with lots of running involved. I stated in an earlier post - everyone runs, you, your enemy, even the mobs run.
With the current state of archery, you can't go wrong with upping your bow skill. For runners this is a very good damage type and is both accurate and fast, too fast to dodge in most cases. Also in small skirmishes softening targets with archery is very helpful. Taking down mounts is also another use.
We see alot of sprinting figure 8 fighting still in melee. One enemy in your stack can wreck you and your guildies, I find it necessary to back off at times and choose my swing times to try to sequence with the figure 8's. One thing you want to avoid is alot of friendly fire damage with 4-5 or more of us swinging at someone strafing us and damaging each other more than the enemy.
The other thing that is important to remember is your number one job in a fight is to stay alive - you cannot help anyone if you are dead and being at 25% or less health is going to make you the number 1 target real quick. Disengage from combat, run behind a rock or a tree or whatever and let the food/pots (you should be using) do their job. Once you are back at 1/2 health or more jump back into the action. You can also pop your mount to help with runners while you are regenning if the fight is to that point. Having multiple mounts is an advanced technique that we see people use from time to time. If you consider they are just steed grass they can add extra life for your getaway.
It is difficult to call targets and the beacon spell, we talked about in other threads, doesn't have much range. Vent is really important in fights and keeping vent clear during skirmishes can be critical.
The number one thing you want to work on is battle field awareness. This is your recognition of what is happening during the battle, who is low hp, who needs help, who is running, how many enemies are targeting you, loc of your party members, your health, running enemies, additional enemies joining the fight, and so many more things I cannot list here. Alot of information is shared in vent and your ability to react to battlefield conditions can really improve your groups chances. Be alert and pay attention, do not get tunnel vision on what you are doing and be ready to switch targets and tactics as required.
In regards to sieges we have been to several. None have went as smoothly as Khosgar. Except Khosgar they are generally a clusterfuck of fighting with no direction and little battlefield organization. Rule number 1 in a siege is still - staying alive. Rule number 2 should be to stick with your party. Generally speaking here are a few of the tactics we employ in sieges, these are typically KGB group tactics since we try to avoid the clusterfuck activity as much as possible. First you will be with your group or you will be dead most likely (I know I repeated that 3 times already). As we enter the battlefield the group/siege leader will be calling a location and someone will be setting a waypoint. This will be the initial fighting point and (possibly) later fall back point if a hasty retreat is called. The first engagement is usually mana/bow range attacks on called target AREAS such as first house rooftop or clan stone or bank etc. As this starts primary fighting locations in town are being assessed and hopefully higher ground can be found, such as a rooftop or tower. Everyone must pay attention and move as a group to each successive location. Tower/rooftop fights typically include shield blocking the stairs (less effective with launch) and group targeting called targets with spells/bows.
One of the other things that is effective in sieges is getting on the clan stone if your on the offensive. Several of the sieges turn into naked fighting by the defenders that seems to occupy the attackers - really what should happen is the clan stone should be cleared by the attacking force then hammers immediately should be started on the stone while squads of attackers clear out towers, rooftops and other higher ground locations attacking the hammer wielders.
If no new target is called stay on the last called target or find a nearby enemy to whack. It's likely that one or more the the target callers/group leaders may disconnect during a long siege and someone should pick up the target calling.
If you are defending GANK everyone if you are attacking GANK NO ONE. Be careful about looting in sieges often we are told not to loot during fights.
If we lose we will likely be retreating quickly to save gear, mounts etc. If we lose and you are naked and want to fight it out its up to you.
I hope this wall of text can help someone.