I have a list and meal plan and I stick to it. We're lucky that we have a Sainsburys and Tesco quite close to each other. It takes a bit more planning, but I make my meal plan and shopping list then using mysupermarket.com compare the prices at both and then split my shopping between the 2 shops. This can save me up to £10.
I also try and make a meal like a lasagne, shepherd's pie or bolognese once a week and put a portion in the freezer.
In the summer we grow our own veg, and the rest of the year and for the veg we don't grow I use the local farm shop or market as it's loads cheaper than the supermarket. I try not to buy too much meat as that also makes the shopping expensive, and what I do buy I try and stretch further. Like if we have chicken fajitas I'll bulk them out with peppers, onions & salad to make the chicken go further.
We're also considering dropping down a brand which is hard as I'm a food snob! However, we used to buy Original Source shower gel and now I get the supermarket own version unless it's on offer. It's half the price. Most weeks we can do our weekly shop for £50 plus another couple of pounds for veg. There's only 2 of us though.
We always take lunch to work too, that saves loads.
We've also recently tried to cut our household bills down. We switched from BT to Sky and saved loads. Just reduced my mobile phone contract by £7 a month.