What makes a good memory turn into a great memory?
First of all, make sure you exercise regularly and get enough sleep. These health habits seem to be utterly important for your brain.
Until about ten years ago it was generally supposed that old brains were unable to grow new neurons. Since then it has been shown that older brains do produce them. You make this happen by increasing the flow of blood in the brain, i.e. by physical activity. As little as three hours of walking, swimming or other aerobic exercise a week increases the number of both neurons and the connections between them. It seems that no research has been done on whether younger brains benefit from exercise. However, if you are young, give your brain enough oxygen, just in case. A detailed article on the topic: http://online.wsj.com/public/article/SB116364034566424589-7dHACZPsmwMK4G...
A good night's sleep gives the brain a chance to process information and to store it in the memory. Sleep is 'vital to update memory', says a BBC news article: http://news.bbc.co.uk/1/hi/health/841058.stm
As for other aspects of developing a great memory, you'll find plenty of web resources if you search for "improving memory" or the equivalent. Pick articles on cognition and memory. If you are currently studying or training, you might want to start by checking out these sites that are written for university students: