We plan our meals a week at a time. Things that are best very fresh (ifish etc) at the start of the week, through to meals I can make from the store cupboard at the end (pasta putanescca works a treat). If you make double and freeze half it normally works out cheaper than doing a different meal every night, and you don't have to buy processed food.
We had our kitchen ripped out a few weeks ago, and it's replacement should be finished today. We've largely been living off microwave meals in the interim. Most seemed pretty nasty, and even the posh 'Innocent' veg pots got dull quickly. I'll feel a lot better when I can get my kitchen