I’m in a contest where the latest prompt is breaking the fourth wall.
Conveniently, in the anthology Stories which I read on my recent trip, there are several examples of fourth-wall-breaking stories. But, without exception, they broke it internally; a woman asking her boyfriend to stop writing her into stories as she was losing bits of herself in them, a man who was offered the choice between staying in his adventure story or living as a peasant in the real world. It worked really well, and is the way I’ve seen it work. Breaking the fourth wall and talking directly to the audience is never something I’ve seen work all that well in a static medium like books and comics.
It’s somehow much less jarring if the layers of reality are internal to the story, so that’s the route I’m going to try to go.
Eileen Young 
The other way that fourth-wall-breaking -sometimes- works, I think, is when the story is presenting itself as 'real life,' so the break remains internal even as the narrator speaks directly to the audience. It's a tricky thing to do, though, without breaking the flow of your narrative – probably why second-person stories are so unusual.