Decide what mental pictures you're sick and tired of looking at, then banish them from the sacred temple of your imagination. Next, browse the fertile depths of your subconscious mind, searching for exciting new mental pictures that you want to install in your awareness full time. For instance, you might want to exorcise a certain fearful scenario that pops up whenever you're under stress, and replace it with a bright, shiny vision of you at the top of your game.
Hm. I'll keep this in mind, but I'm pretty good about doing this kind of thing anyway. Hypnotherapy training and all that.