Like Camdemsmom, I have some set staples I buy every week and then I have a menu built which are the odds and ends.
For example, I get my staples of bread, milk, eggs, cereal and fruit then I add spaghetti sauce (we usually make our own, but haven't had time lately), 1lb of hamburger, 1lb of pork if needed for the meals planned. This usually covers 45-50% of my budget.
Then I'll stock up on whatever is on sale that I can use, like vegetables, BBQ sauce, noodles, rice etc. Especially when Jewel or Meijer has a 11 for $10 sale. Using what is in our pantry already, we have a meal planned out for every night. We do once a month cooking for time consuming recipes like Lasagne & meatloaf, but most of our meals are 20 minute or less meals so we make them every night.
Funny, but our weekly grocery trips seem to get smaller and smaller - I think we ended up spending $37 a couple of weeks ago and that was just on the staples and some meat - everything else I had on hand. DH and I were trying to figure out if we forgot anything, because we were only taking up about 2 feet of room on the conveyor belt and it was our entire week's worth of groceries!
Now, I have learned to draw the line at designating something for each night - that just doesn't work in my crazy house - I tried that before "Wed is meatloaf night" and it never failed that I would forget that someone had something that we didn't get home until late and we would end up making something quicker throwing my whole plan off .... So I gave up on that part.