Would you have a tattoo done? Our sister site WoW Insider has, in the past, posted about people having tattoos of various World of Warcraft symbols done. I sport a tattoo myself, and I've certainly considered getting a WoW tattoo done. Tattooing is certainly an extreme, and fairly permanent, way of professing your love for a game, so I suppose that might be 'too far' right there.
You could go for a simpler option, and pick up some of the nifty official merchandise available for various games instead.
I doubt you'll be surprised to hear that there is a veritable plehtora of merchandise available for WoW. There's the pen and paper RPG, (which I have but can't find anyone willing to indulge me an take part in my campaign) the TCG with its many tasty loot cards, and there's the wearable merchandise. Why not grab yourself a T-shirt or two – my personal favourite is the Camp Taurajo one. Wear one of these around town during downtime and see how many "For the Horde!" or "Grab your sword and fight the Horde!" cries you can hear being thrown your way. I wouldn't bet much on the latter though, to be honest. Sorry Mike.
If your place of work has a 'dress down' day in effect, declaring your allegiance for the entire cafeteria to see may unearth a fellow WoW player you never knew was there.