The problem with any kind of conscious mental activity is that the mind is very limited in the amount of information it can handle at any time. Imagery is an experience or situation from memory; it is kind of like creating your own 'instant replay' in your mind. One thing to remember is that imagery is not 'day dreaming' about the great things you would like to do. It is a specific psychological method that can be used to improve your performance.
Mental rehearsal is best thought of as an adjunct to physical practice. It may be particularly valuable aid at times during injury or at other times when not practicing. For learners, it is probably most effective just prior to performance. Imagery should never take the place of physical practice, but should be used in combination with it. Either before you get to practice, or in the changing room before you go out to start your warm up, it can be useful to go over the drills, techniques and moves that you are likely to be rehearsing.
After practice the feel of the movements involved in the techniques and moves that you practiced will be fresh in your memory, so it should be easier to re-create a clear image of the movements. This will help to cement that image in your mind so that it will be easier to recall during the next practice etc. Even during breaks of play in a game, there is time to create a quick, vivid image of what you should do at the next set piece. You can also practice imagery at any time of day.
Grant Fox, New Zealand's most prolific points scorer in internationals, who was a strong advocate of the use of imagery in helping his goal-kicking performance. Another example is the skier Jean Claude Killy used it to 'run every slalom in his mind' just before sleeping the night prior to a competition.
Top performers in many sports - both team and individual - spend time thinking about their performance and testify to the value of mental rehearsal. Grant Fox, of New Zealand rugby said 'mental rehearsal before and during the game has helped me enormously to concentrate, to focus my attention on the job at hand' this would be a positive aspect of imagery. Where as learners who cannot create clear images in their heads cannot recall and often perform worse as the image is wrong in their head.
Imagery is successful if it used right. The golfer Jack Nicklaus said 'I never hit a shot, not even in practice, without having a very sharp image, in-focus picture in my head. It's like a colour movie. First I 'see' the ball where I want it to finish, nice and white and sitting high up on the green grass. Then the scene quickly changes I 'see' the ball going there: its path, trajectory and shape, even its behaviour on landing. Then there is a sort of fade-out and the next scene shows me making the kind of swing that will turn the previous images into reality.