Sell minis. They're released frequently, and have very loyal fanbases for their varying games (warhammer, confrontation, warlord). The plastic D&D minis seem to be good, as are MageKnight and HeroClix.
Sell cards - people are still addicted to them, and they don't take up a lot of inventory space. Magic, Yugio, whatever.
Background music is good as long as its not overpowering.
Have as many books as you can afford to inventory, but be available for special orders.
Be knowledgeable - amazon knows nothing about games, but they almost always have what people are looking for. You need to be able to beat that.
Keep the store clean, keep your people clean. Smelly help is worse than just being closed.
If possible, provide space for people to game in or on. However, try to keep it seperate from retail space, as no one wants to fight their way past a bunch of angry magic players in order to get to the bookshelf.
Don't wrap books, but don't be afraid to discourage shameless "library" type behavior. Let people glance through, but don't let them READ a book.
D20 is the big boy on the market, but you should certainly include White Wolf, GURPS, and Shadowrun.
Maybe sell other games (board games, etc.) as space allows? RPGs don't exactly pull in the masses. Be ready to answer questions from people that have no idea what's going on, and have ways of bringing those same people into your store - new customers are always good.