Due to excessive computer gaming for several years, I think, I have almost completely lost the ability to visualize, to mentally recall, or imagine, visual images. Have you found yourselves in similar conditions, and through training been able to gain or re-gain the ability to visualize? And in doing so, did you find any particular exercises or techniques to be effective?
'If you poke around online storefronts catering to Buddhist practioners, you'll find these handy little things called "alter cards". They usually have a image of deity or a mandala on them and are very usefull for practices requiring visualization practices, especially for complex images such as yidams, etc.
There are meidtation techniques that help a student learn visualization. They are varied, but one method is that use viualize colors - red, white, blue, green & yellow ride in and out on the breat filling the space in front of you. This isn't as easy as it sounds. Done corrrectly, this practice should involve no conceptualization and it's easy, at first, to develop an inner dialog that accompanies the colors as they arise. There are visualization techniques that can be used with Tonglen meditation - where the being involved in the mediation is visualized seated before you.
In some traditions it's believed that visualizing the guru, seated in vajra posture above your head, at the moment of death creates conditions for favorable rebirth. I've found that practicing this - visualizing the precious guru seated above my head - is a good practice for other visulaizations and can be practiced any time and anywhere.