Combination of advertising, word of mouth, and appealing execution. Take out one part of this equation and you have a game that sells much less.
Just advertising is not enough if the game doesn't appeal or is not executed well enough to create word of mouth (and before anyone says the game is not executed well due to, for example, not reading player movements well enough, maybe it isn't executed well by "core gamer" standards, but just go and read the Amazon user reviews).
Advertising IS critical though, a good game with no advertising will usually not do well either (unless it has adequate shelf space in lots of retails stores, something I believe for example MW:Reflex had).
Currently Playing: Shin Megami Tensei: Devil Survivor Overclocked, Professor Layton and the Curious Village
Anticipating: Xenoblade, The Last Story, Mario Kart 7, Rayman Origins, Zelda SS, Crush3D, Tales of the Abyss 3DS, MGS:Snake Eater 3DS, RE:Revelations, Time Travellers, Professor Layton vs. Ace Attorney, Luigi's Mansion 2, MH TriG, DQ Monsters, Heroes of Ruin
