The training involves getting into the right mindset, clearing the mind and holding it. Basically meditating or staring into a candle. I currently do about 1-2 hours a day. Then I practice by actually playing. Pretty much got it down pat now even with noise and other distractions around me. The hard part I find now is finding a surface that will reflect the number clearly. I play online, but not all of the online casinos have a good surface to focus on.
Try focusing on the laptop/PC screen, first by looking for your own reflection, and then you can use this surface.
Its the same technique you would use when viewing some of those special stereo images you can (Magic Pictures they are sometimes called), as you alter the distance (focus length) between your eyes, and essentially go slightly cross-eyed in the process, the method itself, may also give you that unique time and space to "see" your target number.
Chris