Multi level discount logic ...
As others mentioned there are several ways to implement, here is my 2 cents ...
You can create 2 different levels, level 1 is your normal shoppers and level 2 you can associate all your frequent buyers. Now in your database where you store all your products level 1 gets 10% discount and level 2 gets 15-20% discount.... So when ur frequent buyers logs in they automatically see level 2 pricing which is 15-20% lesser than level 1 ....
If you have still have VIP customers then put them to level 3 and so on ... this way you can expand and still apply your discount rules across all your products easily ...