Just a few things in hindsight that I've learned. Enjoy, comment, discuss your own favorite hindsight lessons and have fun!
Bounty Missions are your friend
Seriously, dishing out a clean 1,000 AP and 200-300 gold + gear? Sign me up. Remember that if you're in a group, their kills will also count towards completion --same for Attack Missions. So share the mission with your group and reap the rewards.
Soul Gems
Yes, the player level matches with that lesser soul gem you have, no that lesser soul gem won't work, yes you need a grand soul gem. Grand Soul Gems are dirt cheap in PVP and can mean the difference between having to respawn and regroup, or reviving teammates and keeping pressure on the enemy. Also remember that if you have trouble killing the NPCs in Cyrodiil, wolves will still fill the gem, and they are easy to tackle even at level 10.
Splinter Teams
Let's say you are fiercely defending a keep from an enemy zerg, a 2-5 man splinter team with a handful of trebuchets that don't mind sacrificing themselves can be a very effective way of repelling an attack.
Step 1: Sneak to enemy keep
Step 2: Attack enemy keep long enough for it to cut off spawns momentarily
Step 3:???
Profit
Uncoordinated Low-Level Bandwagonner's make excellent meat sheilds
You know that group of pre-teen kids that just hit level 10 and are following your group around attacking everything in sight? Yeah, you know who I'm talking about, well it turns out they have a very special use as human shields! Now, I'm all for team playing but if p00nsl4yer and his 500 level 10 companions want to go charging into that heavily defended keep without a second thought? -- Well, better you than me bub, we'll just hang back a sec and clean up the mess when your done.
WW and Vamp teammates - A force to be reckoned with
For some reason, turned-teammates don't really consider this too often (at least on console)...But, the WW and Vamps when working together can be pretty OP. Many of their active skills compliment each other very well, an example being I watched a WW/Vamp guild completely decimate an enemy defending force in the blink of an eye because of how they coordinated. 2-3 Vamps put down volcanic runes and once triggered the werewolves pounced and began pinning and aggroing enemies allowing the squishier DPS based vamps the opportunity to finish them off. Guys, this is ESO NOT Underworld, Vamps and WW need to work together
Last but not least: Repairing
I can't stress this enough. If one of your keeps deep in your territory is miss a wall or two, repair it. Even if you don't want to "waste your points", just repair it enough so that enemies can't just hop over the walls. Here's why: I fight for the Pact. In campaign we were getting dominated by the Dominion, they held all the keeps and the Covenant seemingly ran away with their tails between their legs (per usual). At this point we are pretty much spawn trapped and grinding at the South Morrowind Gate seemingly getting nowhere. Me and a splinter group of 5 however, managed to sneak past and worked our way south toward Faregyl Keep (Pretty far south) when all of a sudden we noticed HEY not only is the south-eastern wall completely down, but so is an inner keep wall...not only that but it holds a scroll! Seriously, a group of 5 stealthed our way into this keep, stole the scroll and made off with it. Now, did we get back north with it? No. BUT, it distracted the Dominion enough between Faregyl showing it was under attack and a big fat notification saying "Your scroll just got jacked" was enough to get the Dominion zerg to refocus back south long enough for everyone to finally gain some ground in the north.....Don't do that, repair your keeps, at least partially.