When buying loose veg, like leeks or mushrooms at the supermarket with the self service payment teller, just put the bag on scales and push button for onions/potato's which are the heaviest.
I can get a half kilo of mushrooms for 60p instead of �1.80p.
Eat before you go shopping as if you are hungry you will be tempted to buy biscuits, chocs etc - also, make a list of what you need and stick to it (not literally of course).
trt that really made me laugh but they check it out in our supermarket; what I do see people do though is weigh out their fruit or veg then fill the bag up further before closing it.
Check out auctions sites like ebay for items ending soon which have no bids - a lot of stuff remains unsold at a starting price of a few pence, and it's not all tat. I've noticed that an identical item can have a few bids one week, and none the next. There was a web site which highlighted unsold stuff ending soon for pennies, but I can't find it at the time of writing this.