Stock up on your basics. Pasta and rice are filling and cheap. And you can liven them up pretty easily. Buy some cous cous and quinoa for a bit of variety.
Market Fruit and Veg
Find your local market for some cheap fruit and vegetables. If you buy something supremely cheaply that only has a few days left in it, you can make pasta sauces, soups, casseroles etc and freeze them. That way when you are feeling hungry and lazy you just have to warm them up.
Herbs and Spices
Shelling out for herbs may seem extravagant when you are on a budget, but plain pasta gets boring very quickly. Once you have bought the herbs they are ready for you to use time and time again. Something like half a tea spoon of curry powder can liven up an ordinary sauce. You can even grow some of your own herbs. Chives, for example, can be grown on a windowsill and the seeds are very cheap.
Baking
Baking cakes from scratch can cost more than buying ready made, especially if you need to buy all the ingredients. But you can make bread very cheaply. All you need is flour and sugar and if you buy a tin of yeast, rather than sachets, it will last you ages. It's not too tricky and nice home made rolls make a pleasing change from economy loaves.
Economy Ranges
Experiment with economy ranges. Avoid the temptation to by loads and loads before you have tried it. It's not a bargain if your tuna tastes of fishy sawdust and you can't chew the beans. You may find you like some economy items, which is good. Also try shops such as Aldi, Lidl and Netto. These shops generally have far lower prices without the obvious drop in quality.
Be Creative
You may reach a point when you have only a few items left. Try unusual combinations, since there is nothing worse than eating the same meal day after day. Look on the internet for recipe ideas or check the library for cookery books. Just avoid the temptation to buy in lots of new ingredients!
Multibuys
Multibuy offers, like three for two, can be great if they are on something you would buy anyway. But be careful if you end up buying three bottles of lemonade when you only need one. And look at the offers. It may be three bags of pasta for 99p, but would it be cheaper to just buy a large bag?
Treats
Make sure you buy yourself something you would like. If you have a sweet tooth, generic angel delights and jellies are very cheap and make you feel less cheated by a simple meal. If you need a chocolate fix, buy a large bar and as soon as you get home, split it into sandwich bags. Eat a portion of chocolate when you feel you need to. If you can afford to, have one minor extravagance. If you really like coffee, buy a nicer blend. It may seem contradictory to spend extra money when you are on a budget, but we are human and we all like a little treat.
Water
Try drinking water instead of juices or fizzy drinks. It is good for you and will save you a fortune. If you cannot stand water, then squashes are a cheap alternative. It will help you re-hydrate and over a few weeks will save you money you can spend on something more deserving.
Try Shopping in Different Places
If you have an Asian market near you, then have a look. You will find bulk packs of rice and interesting spices and pulses. Don't be afraid to try new things. You may find a number of the things you already buy are cheaper than in supermarkets.
If this isn't an option near you then try different shops, that may have a range you haven't tried before. You don't have to buy all of your food in one place.