This is something I do often, but I haven't seen it posted here(might've missed it), and so figured I'd share for anyone who doesn't do it.
I like to wear poker t-shirts whenever I can, I wear them a lot. If you contact some of the poker rooms and mention that you play in live tournaments or hang out at bars etc, they might even hook you up with a t-shirt or two for free.
While out and about, I inevitably get spoken to by someone who also plays poker online, whether it be for play or real money. If you've been in this situation, then you *know* that this is how the conversation usually goes:
"Oh so you play poker?"
"Yeah"
"Online?"
"Yeah"
"Me too, oh man I play on ____ Poker and last night I had this horrible beat."
They'll then start to tell you about the horrible bad beat they had, and candidly mention how the room is rigged against them
These people are our prospective customers, and instead of just chatting with them for a few minutes while thinking "I hate these stupid bad beat stories", you should be using them as an information source. I personally don't try and hit them up right then and there about switching to another poker room; I talk to them, and I find out what made them sign up for a particular poker room. Once I understand why, I find out as much about their demographic as possible, so I can plan to market to them offline in the future. Good ol market research.
But I then go one step further; I say that we should play poker online together sometime. Everyone's usually interested in that, and you swap e-mail addresses, AIM or MSN screennames etc. Not only can you gain a new friend to talk poker with, but the possibilities are endless. You can arrange to play him at a specific site via your tracking code, you can get him to have private tournaments with HIS friends that you set up for him, again getting them through your tracking code, or if you think they're smart or enthusiastic, even approach them about being a sub-affiliate, or doing marketing for you on the side. The possibilities are endless.
If you're not comfortable wearing poker t-shirts, there's lots of other options; poker hats, key chains, wallets, watches, hell carry a WPT or WSOP DVD set around with you sometimes!
Also, and this helps if you play poker professionally so you don't have to lie

but be sure to go get your haircut every couple of weeks. We know how hairdressers like to talk, and when they ask you what you do for a living, you state that you play poker, full-time or part-time. They ask online, then state that "I/my brother/my sisters boyfriends daughter play online poker too", and when the haircuts over, you give them a decent tip and hand over a flyer or too, and tell them to "check this site out"
Just some ideas I wanted to share. You can put out as many flyers and business cards as you want, but nothing is as strong as word of mouth.