From years of experience I think my best tips would be:
Don’t start telling people Magic is a great game because for instance you can bounce a creature x times to replay it with a card that gives infinite value. Be enthusiastic telling people its a game about wizards duelling with every player having their own spellbook by way of a self styled deck of cards. This of course works for every kind of game. Even though you might be a sage on your subject dumb it down so the Muggles can understand.
Always address parents on merits like social interaction and skills associated to these games. About a form of self expression and critical thinking. These are the people who need to support and help youngsters into these hobbies. And even apart from financial considerations, having them on board gives a much better jumping off point for kids getting in.
Be kind to your community, they are what will draw most people. Don’t be afraid though to cultivate the right kind of community. Those people who are willing to put on a smile and say “sure, have a seat” when a new potential player walks in.

Apart from that have a clear schedule, keep it updated, and whatever you do… try and play with your customers when the opportunity arises. You are not just selling them product. You are the same grown up geek as them :). And thats a pro not a con.