How do ask your wife to do anything? The answer is usually, "very carefully." But it's surprisingly easy to spend less money each month, especially if you both understand where the money is going!
The first thing to do is track your monthly expenses. There's software like Quicken that's specifically designed for tracking household expenses. (And it can even let you pay those bills online automatically each month.) But even a simple spreadsheet can tell you a lot about where the money is going. Excel has a feature where they'll convert any set of numbers into a pie chart, which can sometimes create a very dramatic picture of what's wrong with your budget. If entertainment is consuming too much of your income, itll become glaringly obvious when you see it represented as a glaringly obvious, enormous, brightly-colored pie wedge!
But the important thing isn't the spreadsheet - it's just becoming aware of how much disposal income is left at the end of each month. If you have a whiteboard where you leave messages, you might want to try jotting down a quick estimate where you both can see it for a couple of weeks. ("Monthly income: $2400. Mortgage payment: $900. Car payment: $250....") The purpose of the spreadsheet exercise is just to identify the "problems areas" for overspending. Sometimes there's even a few obvious monthly expenses which can be eliminated right away.
Unfortunately, credit cards make it easy to overspend. The purchases are made, but their cost isn't actually deducted from your checking account (and your monthly budget) until the bill comes due, many months later. A simple way to address this is to keep a running tally of how much is being spent on the credit cards. Try it for a month, and you'll learn just how quickly those small purchases can add up to big money! (And if you don't want to keep a tally, the next best solution is to go online periodically to review your current credit card balance!) Unfortunately, sometimes it can take a while for new purchases to get added to your current balance. That's why some financial advisors recommend switching to cash for purchases whenever possible. Then there's no surprises later in the month, when the grand total comes due for all those little purchases!
It's an unfortunate cliche that women like to shop and spend recklessly. The truth is that women are just as smart as men, and can spend less money each month - once they understand where it's going!