Helping a spouse become more financially responsible

How can I encourage my wife to be more thoughtful about money? Our family has really struggled recently. I was laid off from my job two years ago and have finally found full-time employment. My wife worked a couple of part-time jobs, but we still have a lot of bills to pay. And no matter what our income situation has been, she's always spent money we don't have. She'll buy herself and our kids top-of-the-line clothes, even if it means we won't be able to make ends meet. I've tried to discuss this with her many times, but it doesn't seem to matter.