It depends who is the speaker. I'd think that if someone other than yourself was placing a suggestion on you, it'd be second person. If it was true autosuggestion, you'd use first person. It's the decision one intuitively makes, so maybe it is the most successful?

There was a study by Ninja, I think, about subliminals (here). It concerns first and second person commands (among other stuff).

Subliminals, however, are not the same as autosuggestion. It would make a good study!