I've got some tricks up my sleeve and there's no reason I should guard those jealously for myself like a marketing dragon, so here goes
Would you ever develop your game without a plan? No GDD, no direction for the story, atmosphere or gameplay? The likely result wouldn't be so pretty, and you know it. And marketing is no different: it's not like whip-up magical stuff. And for that very reason, you actually can promote your game yourself if you're smart about it.