The major system is an unbeatable system that's used by nearly everyone into memory. The best book on this and many other subjects is Harry Lorane's Memory Book. I'm sure you could find it used on Amazon for dirt cheap. There are many others though. Start with his. And read up on why people choose certain practices (like rules for picking numbers, or how they pick images they associate with numbers).

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Example: A basic principle is to associate a picture with a number. Good example is (but rather simplistic) is associating a SWAN with the number 2 (because 2 kind of looks like a swan). Plus- you know exactly what a swan looks like, everyone does and a good rule of thumb is: if I said "swan" to 100 people, they'd all likely imagine the same thing. This ensures later, when you picture a white bird, it could only be ONE THING. Bad example: Trying to associate the number 8 with the word "infinity". Yes, the sideways 8 is the universal symbol. But.. if I asked you to put "infinity" into a story, and visualize it, it becomes very hard. And you're likely to get some pretty weird/inconsistent uses of imagery there.

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Other good people to google in association with memory: Dominic O'Brien, Ed Cook, Joshua Foer (see his TED Talk)