This was actually one of my new years resolutions, so I actually just started this year. It has saved us SO much money. I used to use coupons a lot, but I've lost the time for clipping, organizing, etc. However, meal planning helps save a lot. Our grocery bill for just the 2 of us went from about $250-$300/month to $150-$200/month, including stuff like shampoo and laundry detergent.
I probably spend 30 minutes a week on it. I am online a lot, so I gather recipes randomly throughout the week. Then on either Tuesday or Wednesday, I sit down with that weeks ads from our grocery stores. First I look through all of them, circling what is a good deal and what I want to find a recipe to use. Then I'll try to think of or find recipes for stuff that is on sale and based on what I already have. The key for me is making sure most of what I'm making and shopping for is on sale, this saves a ton of money. What we eat usually depends on what is on sale.
I have a little spreadsheet print out that I use now, but I didn't use it until about a month ago. I write down 6 recipes I plan on making that week. Then I go through what ingredients are needed in my head and write down I need to buy based on store.
It sounds a little intimidating and time consuming, but I really don't spend more than 30 minutes on it each week.