According to this PhysOrg.com story
http://www.physorg.com/news65283028.html
subliminal messages work, and work best if the information offered meets a felt need in the observer.
It occurs to me that including that explicitly may improve self-rolled scripts by linking the desired effect with a need (e.g. to become more confident, to lose weight). As long as the connection is clearly made, I'm not sure it even needs to be completely logical. For example: "Larger breasts will give you the confidence you crave." I suppose the need can even be manufactured to some extent with guided imagery.
Note the above is pure speculation from a relative newbie just wrapping (warping) my head around all the ins-and-outs of hypnosis.