Plan a weekly menu
Sit down and work out exactly what you're going to cook for the next week so that you know what ingredients you will need.
Write a shopping list
Write up a shopping list based around your weekly menu and when you get to the supermarket, stick to it - don't be tempted to impulse buy and end up with a cupboard full of food you don't need.
Shop when you're full!
Never shop on an empty stomach - you'll be far more tempted to buy things you don't need if you do.