I try to think of what bags I have and want and try to get a wallet or two that match all these.
For example, I recently bought the tattersall french purse...this matches anything bleecker, my new patent ergo that's coming my way, and the new heritage stripe lining.
But I also have the hamptons french purse in mahogany which also goes with almost everything.
If something really knocks me out, I will get the matching wallet otherwise I try to conserve the $ for something else.