Hypnotists are encouraged to have a special view of the unconscious mind. It is suggested that the unconscious mind holds all experience that isn't consciously available. This also means that the attempt to improve your memory becomes an attempt to improve you ability to recall. This is stimulated by research showing that electrical stimulation of the brain can make a person recall something they had forgotten, right until the electrical stimulation stops, then the memory is gone again.

You may also have seen television shows about "savants" who are persons with exceptional mental abilities (made famous by dustin hoffman in the film Rain Man). These people seem to able to remember everything at the expense of social skills. It's a common belief in the field of hypnotherapy that everyone has a subconscious mind that contain every experience of our life, and that is very capable of performing tasks that are way to complex for the conscious mind.

I don't believe that memory is a constant that you cannot affect through your life. It depends a great deal on what you want to remember and your mental strategies for doing so. A hypnotist or an NLP practitioner can help you develop strategies for the things you would like to be able to recall. Keep in mind that you should have a purpose, something that you would like to accomplish by increasing your ability to recall. There is no point in training your memory just to have a better memory. When you know what it is you would like to remember better, you can seek out someone to help you increase your ability to recall.