Reading books is number one. Flame meditation is the greatest meditation to visualize or have visions (I think Cobra put it under elemental divination but can be done without any element infusion for beginners). Pointed meditation, same as the above. Color meditation also helps a lot. If you have time, even drawing.
You need to achieve stability, then sharpness of mind and focus, then accordingly increase of visual faculty and imagination. The above help that.