Heehee, I approach this problem from the other side: I loathe shopping, and would prefer to go for months without buying anything. I have a closet almost entirely of neutrals, and even so I'm missing some basics to make my neutral wardrobe sing (eg, belts!).
I talked to a co-worker who had the same problem, and her solution was to set a budget minimum for herself. Her monthly minimum is a dollar amount (I can't remember what percentage of her income it is, but Sally @ Already Pretty has a good guideline) because she buys everything new. Personally, I try out new-to-me looks at thrift stores, however, so my minimum is based on the number of new-to-me pieces. It's still too tough, though ... I don't think I've bought anything since the silk skirt I posted about in November.