Mindhacks blog today has a great synopsis of some research on post-hypnotic amnesia. It's most commonly used when the subject is given a suggestion to forget part of the session.
From my perspective, it's most useful when there is emotionally strong content or when there might be conscious resistance (but subconscious agreement) with a stated goal. I never use post-hypnotic suggestions to forget things outside of a session. I do use hypnotic awareness training to "simply not notice" things. A common example would be to not notice the candy dish at the office. People can still see the bowl, but they don't notice it. It sounds strange, but it works. The hypnotic suggestion I use in such a case either creates context reframing so that the candy bowl means something different than it used to, or it actually trains attention elsewhere so the candy dish is always in the background.
Think of the famous image that flips back and forth from a grail to two faces facing each other, depending on how you focus your eyes. It's a figure-ground shift. We have habitual ways of perceiving things that are usually subconscious...i.e., we tend to either see the grail first or the two faces (most people see the grail). Hypnosis can help change those habitual patterns of perception. There is a saying, "where attention goes, energy flows" so it's good to have more choice in what you pay attention to and what you ignore. To me, the ability to re-train awareness is one of the most powerful properties of hypnosis!
