When everything is going according to plan: Do a week meal plan, shopping list, then shop. So I know what I am having every day for a whole week!
When going bad, pop to shop EVERY afternoon and buy one meal :(
For my meal plan, I sometimes get a cookery book or magazine and flick through that, or sometimes try and use up stuff in the freezer, other times I just think "1 meat, 1 pasta, 1 fish, 1 veggie", come up with meals based on that, and then try and double up stuff - like this week Monday was fish n chips in the oven, so I got a mince out the freezer and made a quick meatloaf and cooked that at the same time - so Tuesday was spagetti meatloaf-bolognese (very easy and saves pennies not having the oven on). Thursday is roast chicken so Friday is chicken fried rice. You get the idea!
I have been known to buy a fair few ready meals, but can't do that now I am baby-led-weaning a 6 month old (too much salt) - so now I'm deciding which ready meals I might fancy and making them myself. I also make double and freeze (if M&S can freeze it then so can I!).
I shouldn't say this, but I have been known to thow some bernard-matthews turkey nuggets into the oven to feed my v fussy 3 year old. Its waaaay to difficult to cater to a fussy husband, 6 month old and fussy 3 year old EVERY meal.
Also not sure if I should point you to another site? forums.moneysavingexpert.com - the old style board. Every week there is a post (search meal plans on the old style thread) where people post there meal plans for the week. Good for inspiration, or if your just nosey!
Hope that helps.