This may sound too obvious, but I've found that FULLY AUDIBLE messages (that is, affirmations), repeated in a loop, especially prior to the hour or so before sleep, have much more power than subliminal ones, since ALL areas of my mind can "hear" them (even if only in the background), especially if done in a musical way; that, and the fact that the memory of the soundwave automatically repeats to the point where you can actually HEAR IT when it isn't playing (due to persistance of vibrations in the inner ear or whatever). It doesn't work so well AS you are sleeping (at least for me) since all I dream of then is to find the source of the sound and turn it off! (This does NOT apply to altered-state meditation where it DOES work for me.)