I think it depends on whether you're trying to recruit existing pool players to your room or whether you're trying to bring in new people to play.
The first one isn't too tough to figure out, you just have to make it a place where players like to play. Good prices, well kept tables, etc. I think membership cards, punch cards, etc. are all great ideas. I don't play as much as I would like in part because of cost. I wish the place I play had a membership option where, for a certain fee I could play all the pool I want during designated hours but then get a discounted rate during the really busy times likes on Friday/Saturday nights. This would let people play when the the place isn't super busy anyway (weekend days for example, I played for 5 hours yesterday and less than half the tables were occupied). Plus, if you're in an area with multiple rooms that will keep your clients loyal.
Attracting new players is a different story. In my experience most pool rooms are not the kind of places someone would go if they don't play pool. They just aren't that nice and don't really draw people in. The place I play is an exception to that, it's at least half focused on the bar/restaurateur side of the business (probably well more than half actually) and it gets people coming in for drinks, to watch games, families come for lunch, etc. It's a cool place, it's really nice inside, and they have quality food and drink. I think this helps bring in non pool players who then go play for fun after they eat sometimes.
In trendy cities there has definitely been a movement to activities similar to pool...I know of hip bowling allies in NYC that are always busy, places with lawn bowling on the roof, bocce ball, etc. They are frequented by trendy hipster type crowds of young people who dig things a little outside the main stream. I think this is a market that the right pool hall could definitely cater too, but you have to be in the right area and have the right kind of atmosphere. It may not make for the best players type of room, but it would definitely get new people in and playing I think.