I just uploaded the 2009 January-May dinner menus to our website. I plan out our meals 2 months at a time. It saves so much time (both at home and at the grocery) and money, that I can't ever imagine going back to shopping more frequently.
When Bryan and I were first married, we used to shop every week. After the first baby came along, shopping no longer seemed like a date, so we started going every two weeks. By the time baby #2 was born, we had to drive further to get to a Super Wal-Mart, so we transitioned to every three weeks, and shortly thereafter, once-a-month. A year ago I proposed trying every two months, and we've never looked back. Start small, work your way up to it.
The first thing I do when I sit down to plan is to write on the calendar all the activities that we have going on. This helps me to identify the nights I have more time to cook vs. when we are on the run. It also shows when I don't need to cook at all due to Bible study nights (when we pitch-in for pizza) or out of town, so I'm not buying more food than we need.
When planning the meals, designate each night of the week with a different theme. In the past I've used "breakfast night" (a great budget-stretcher), "Crockpot night," "new recipe night," "Mexican night," "quick and easy night," "things with frozen meatball night," etc. We eat our leftovers for lunch, but that could easily be one of the themes. Then I fill in down the column what fits in with the theme. It's OK to repeat! Especially with breakfast night, I often use a three week rotation.
I love to try new recipes. I'm forever in search of new ones that will become family favorites. So my menus are typically a mix of new recipes and old standbys.
For breakfast, we have a set menu that does not change. That way I know that I need to buy 9 weeks worth of English muffins/bagels/toast (Friday's breakfast). I prefer reheating leftovers for lunch. It's usually faster and more cost-effective to make more at dinner, than to re-invent the wheel for lunch. We do keep pasta and PB&J on hand for days when we don't have leftovers.
The our food budget is the biggest expense that I can control. By investing a few hours every couple of months to plan, I can dramatically keep our costs down.