I learned a good system when I started buying BPAL.
Usually I stick with limited editions so I buy when I buy a limited bottle, and then I pick up one to two additinal bottles or samples I'm interested in. If I don't find any limited's I'm interested in, the when my primary supply runs out, I reorder what I have run out of, and an additianal 1-2 bottles/samples. That keeps my list limited, my budget under control and slowly but surely builds my list of favorites.
I fully plan to do the same with this, although I will be addiding attys, and cartridges with every purchase to stockpile those, as well as my liquids.