I started a group in my area about a year ago and it's still going strong. here's what I did:
First I started a yahoo group so that the people who join to post questions, etc. to each other and so that I could manage the meembers by sending out notices for when we had scheduled meetings. Next I made up a few flyers and hung them up at a coffee shop and since I work for a school system--I put them in the teacher's lounge of some of the schools. Next I kept some flyers on me and (this was the hard part for me b/c this is totally out of character for me...) when I saw people looking at yarn in Michaels, etc., I have them a flyer w/ all of the info on it--my number and the yahoo group. I first wanted to hold it at a nearby coffee shop but found out it closed really early in the evening and it wouldn't allow enough time for most folks who work so we found a coffee shop that was just opening in a town nearby that's not too far away. I called the owner and she said we were welcome anytime--I scheduled my first meeting a few weeks out and posted it on the yahoo group. I also posted a message asking for suggestions. It started with just two or three people (one of which was my mom, a crocheter) but people would see us in public and stop and ask how to join. Shortly there after people started telling friends and they started to come. Now I have about 35 registered members on the yahoo group and about 12-15 of them participate regularly in person. Recently, the group has selected another coffee shop location that has just opened b/c it stays open later. There is another group in our area that meets at the local barnes and noble too so bookstores are also a good idea. currently, my group meets every Monday evening from 6pm until ? (10 is when the Caribou Coffee closes) and we meet every other Tuesday evening at the starbucks cafe inside of barnes and noble. We have one meeting once a month at a kid-friendly location so that people w/ young children can still come and participate. usually we meet at a park or a local "play gym". When I go on maternity leave--we are talking about starting up a daytime group for those who don't work. My group runs well now and I don't have to always attend. For the first 6 months or so I had to be committed to attending every session and making sure I kept conversation flowing, etc. I'd often bring knitting books and magazines for people to look at and talk about and I'd offer to teach newbies how to knit. Now I miss a session each month b/c I'm room mom for my daughter's class and we have meetings and if I'm tired, I don't feel bad if I don't stay the whole time. The group still runs smoothly--even when I don't attend!
Good luck--check out the section in Debbie Stoller's first SNB book--it has a section on how to locate/begin your own SNB.