This article from MSN money is fantastic. Basically the state of your wallet is the state of your finances on a very small scale. So, if it's stuffed with crumpled bills and receipts your head is in the sand but if your bills are orderly and you know how much is in there you have a pretty good control of your money.
It's not the first time I've heard this either, how you treat your money is how you TREAT your money. If you're disrespectful to it, leave it scattered around and generally aren't aware of it then you are probably trying to sabotage your finances or "don't care" about money. I say "don't care" because like it or not we all care about money.
Since I learned this principal I went from the crazy messy wallet to the organized wallet and wouldn't you know my finances improved dramatically?
Here are some good tips to slim down your wallet.