To quote from my first book on the occult, Essential Wicca (By Paul Tuitean and Estelle Daniels);
Take a candle and place it a safe distance away from you, but close enough that you can see it clearly. Study the flame's movements for five or ten minutes and close your eyes, seeing the flame move as it was (and is) in your mind's eye. Open and close your eyes until you can see the flame move perfectly in your mind (not just imagining it, but seeing it when you close your eyes).
This is one of the two techniques given, but it works. The point of visualization is so you can manipulate your own energy better and tell your subconscious mind what your intents are.