Alright, I'm no Warsong Gulch expert by any means, but I have a tiny little plan for my flag running scenarios. It's simple, but each scenario changes depending on how the other  team's behaving, so it's kind of inconsistent. What it takes most of all is hoping you're not alone, and the guys with you helping you get that flag are thinking of a similar scenario on how to get you out of there. 

At the start of the game: As you're running to the flag, check the field and take note of the alliance running towards your base. You can get a feel for how many are on defense initially. It's like countin' cards.

As a shaman, the tunnel is something I use very rarely, as I try to take advantage of as much terrain-in-ghostwolf as I can. My first options would be the ramp, followed by the graveyard. I try to stay far to the west or east, keeping obstructions between me and line of sight of midfield players searching for me. After I'm halfway, I check my map for close horde players and attempt to converge on the strongest bunch of them. Unless they're in a midfield battle, I stay the hell out of there and use it as a distraction. 

As a druid, you're potentially the best flag runner, in my opinion. All's you have to do is be ready to reshapeshift each time you get a movement impairing effect. It's very hard even with frost shock cooldown and earthbind cooldown to prevent a druid from getting away sometimes.

The tunnel is a very common return point especially if the flag runner is accompanied by protectors. It puts everyone on even runspeed ground (potions aside). I don't like the tunnel..because I don't like to be on even ground! I only use it if I know a big bunch is respawning at the graveyard or if (by checking the map) alliance are already outthere awaiting my suspected arrival. 

It's also mainly key to be in there with players who know how best to help you with their class. A warrior with a warlock/priest can break up a heavy defense with their fear so that the runner can get out, and the priest shield is huge. Mages going in and sheeping (with their fellow attackers knowing not to break/dot the sheeps) helps too.

And then of course, there's the equipment level of the teams. You get to notice the names of the players on the other side and recognize if your equipment is up to par to face off against them. Unfortunately, on average, Alliance battlegrounders in arathi basin and warsong gulch are better equipped than the Horde teams.

This has all mostly been rambling. I considered just deleting it, but  awwww to hell with it.