I try to go grocery shopping just once a week, but there are always those weeks here and there when I have to make extra trips. On a good week, I do all my shopping on Friday. So usually on Thursday I take out my family calendar and on my work calendar and look at the week ahead. I look to see which days I'll be working in the afternoon, which days my kids have activities, which days we might have a meeting to go, etc.
My first step in deciding what meals to make involves choosing recipes based on time and method of preparation. If it's a day when I'm working and we have after-school activities, I know I won't have time to make anything around dinner time, so I either choose a crockpot recipe or something that I've previously prepared and frozen and can just reheat that night. If my 12 year old daughter is going to be home, I might choose a meal that can be mostly prepared in advance but that she can just pop in the oven for me at a predetermined time. If I know I'll have a little time, then I can choose a meal that might take a little longer (but never more than 30 minutes in my world!) to prepare. I also plan the nights that we'll be eating out.
My next step in the meal planning decision making process is to vary the types of proteins we're having throughout the week. I try to have one meatless meal each week and I don't make beef or pork more than once a week. The only thing we might have twice in a week in my family would be chicken or pasta. I also ask everyone in the family if they have any particular cravings or meal requests for the week ahead.
The other key step in the process is that I write it down. Since getting a stainless steel refrigerator my magnetic surface space is limited so I just write my meal plan on a small scrap of paper. I post it on a board near my phone and make sure I look at it every night. If something needs to be defrosted, I make sure to take it out of the freezer at least one night before (but 2 is usually better.)
My way is certainly not the only way. What's important is that you try to find a way that works for you and your family. Hopefully, my process might give you some ideas to develop your own.
I also wanted to share a link to another great blog, RaiseHealthyEaters.com, which recently ran a series on meal planning with some excellent suggestions.