I have finally reached a maturity with my main group (Pal, Priest, 3x Mages) that I can actually contribute to winning a BG. I don't top the DPS meter, but my priest usually leads healing, and my mages are 3,4 & 5 on dps or so. I've had some moderate luck in WSG lately, but the strategy I've been using is different than the defensive strats I've seen posted.

Basically, I have found it very helpful to play offensive, but not front line. I will usually follow a warrior or feral druid into the enemy flagroom and help keep that player up and secure the room while they capture. I will then escort as best as I can, while burning down chasers and interceptors. Once the flag runner is back on our side, I go back on the offense to get back our flag, usually with the help of a rogue or hunter. Using this strategy, I find myself in the enemy flagroom more than anywhere else, and I've had a number of wins with this strat. Playing defensively, it always seems like I deter them from attacking until they have the numbers to take me out. By assisting the offense, they can't focus on me, or they will let our flag runner through.