It seems that a combination of putting on live performances, as well as hosting lectures, does a lot to build membership and bring in younger members. At least, that's what the Fresno Magic Club has done.
When I joined it was small. 6-10 people per meeting was normal. Most of them were two to three times my age at the time.
Over the past few years, largely due to my friend Kyle (who just released his first product to the magic world, GripT), they've been doing a lot more public performances. They sort of partnered with a winery to do periodic member showcases, as well as a now-annual performance at a dinner theater in town. Which, I might add, they sold out faster than the dinner show's regular entertainment sells. So lots of people in the area are seeing these people perform and have fun. They also started hosting lectures by decently well known magicians, so there's incentive to attend and be part of it above and beyond watching everyone in the club do their favorite card trick for the 22nd time. From what he's told me their membership is consistently 20-25 folks with another dozen or so who come and go.