This has only been tested with a rogue.
In Warsong Gulch, you can easily ghost the flag.
Anyways, go up the tunnel to the opposing faction's base. There's the small cave with the golden boots (uber speed) that nobody really looks in to check for stealthed rogues.
1 Turn off stealth, wait until you have 1-2 seconds left on the cool down timer.
2 Run over the golden boots. (You don't have to have the boots but it makes it ALOT easier with the speed).
3 Turn on stealth. You can hide next to the cave wall if somebody is coming or you know allies are in the flag room and might run by.
For players who can stealth: I'm not sure how WoW works with autorun (just came from eq2 a few weeks ago) but if you don't have stealth, I would imagine you can get the golden boots and turn autorun on, facing out of the tunnel(the way you came in) then turn your modem off to deceive the opponents into thinking you're running off when your toon is really stationary at the spot where you turned off your modem (so they get out of range errors, blah blah). After you turn the modem off, stop and go get the flag (So they can't kill your toon where you originally turned off the modem). Don't run too far down the tunnel before turning off the modem or the flag won't render.
4 Turn off your modem.
5 Run to the flag, get close enough where the cursor turns gold and click the flag.
6 Run out of their base.
7 Turn back on your modem. Boom you get the flag. (I usually could make it to the hill before I had to turn the modem back on)
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There were alot of allies at the GY in the video so i did two 2-3 second modem toggles while running across the battlefield to mess them up.
Video Note: The "Turn modem back on" caption should be a few seconds earlier. I'm not sure why it's late, but it's pretty obvious when I turn the modem back on.