I thought this achievement would be much worse than it was, but it turns out horde hate it just as much as the alliance and vice versa. What this means is that it is more than likely that some sort of mutually beneficial if only temporary truce will be present at the start of 3 of the 4 battlegrounds.
Eye of the Storm is a free for all against not only the opposing faction, but your own as well so everyone is on their own there if they didn't bring their own help.
So in the spirit of cooperation here are the rules of engagement I discovered doing this achievement yesterday evening. First for returning a flag in Warsong Gulch, let the opposing team take your flag unopposed as they may be trying to help and drop it right away, if they leave the room with the flag (or try) then they should be eliminated. Also if the opposing faction comes in too great of numbers all bets are off and assume the worst. Once the flag is dropped everyone should be nice about taking turns returning it and if you have already done it run over to the other side and extend the same courtesy to the other side. Keep in mind that not everyone knows to right click the flag buff to drop it and some players will be using an ability such as stealth or feign death to drop a flag, so don't beat on them if they don't drop it immediately.
Once everyone has returned a flag that needed to, or enough people get restless the truce ends and the battleground proceeds "normally". I noticed even though we were still competing for a win that there was still a cease fire when players weren't fighting over a flag. You could still walk around freely in the opposing faction base without being attacked so long as you minded your manners.
Next up is capping a tower in Alterac Valley. There is going to be a race among your own faction to run to a tower and cap it before everyone else. This is a little annoying as it is very easy to fall off a spiral staircase when you are in a hurry. No worries here though, the opposing faction will likely come and help you out. Like Warsong Gulch allow the opposing faction to retake the tower freely so long as your faction overwhelms the horde force so that if a fight did break out you would surely win (even those with good intentions can change their mind when odds are in their favor). Once the flag reappears again everyone takes turns capping it until everyone is done. This is going to happen at towers closer to the middle and those who have already done it or aren't interested will head to the ends to down the bosses. Once you are finished it would be nice to head back to your own towers and help the opposing faction in the same way.
This truce really only applies to the central towers and also somewhat to the roads (again in the central portion of the map) as single runners could be going to help. The far south and north are business as usual and by usual I mean a partial turtle as both sides should probably have a few defending their bosses to make sure they don't lose while all this good will is taking place in the middle of the map.
The last "easy" one is assaulting a flag in Arathi Basin. The battle starts off with your own faction racing to assault the flags first, then for a time the battle is the same as any other Arathi Basin until the flags are captured. At this point the truce begins, allow the opposing faction to approach and assault your flag so long as they do not come in a large number to overwhelm the group you are guarding the flag with. The opposing faction gets credit for assaulting your flag and it is immediately defended by someone clicking on it again. At the same time if you need to assault a flag you walk up to an enemy controlled flag and assault it without attacking anyone. If you are attacked then try another flag. There will be some players that will just try to be mean, but it's really no skin off their nose since the flag is only lost for a second or two and hardly effects the outcome of the battleground.
This truce will likely last the entire duration of the battleground, so long as you walk up to an enemy flag in a non-threatening manner you probably won't be attacked.
The last one is capturing a flag in Eye of the Storm and this is totally going to be a grind. Both your faction and the opposing faction, 30 players in all likely want to grab that flag and return it to their base. The center of the map around the flag spawn is going to be brutal and maybe half a dozen people will actually get to do it any given battleground if they are lucky. It is possible that a battleground could end without anyone capturing a flag.
In this case I left it up to Karma, I tried my best to grab the flag and when the opposing faction was trying to grab the flag I fought them off rather than try to grab the flag myself. I got lucky and the third flag spawn in my second Eye of the Storm I got it and capped it.
My next step would of been to recruit some help to fend off the opposing faction (and possibly my own in the form of a pally bubble canceling the flag pick-up cast).
Good luck out there! When it feels hopeless keep in mind that the other side is just as frustrated and possibly annoyed as you are and will probably try and help you out for a favor in return if given the chance.