This will depend on where you live, but if there is a good market locally, visit towards the end of the day. There are lots of bargains to be found.
When DD went to university she turned a shopping habit into a bargain hunting habit. She was fortunate to be at university in London and living in Lewisham, which has a fantastic market. On Saturdays she would go to the market at 4.00pm and then ring me up with stories of buying 14lbs of onions for £1, stored in a box outside her back door, luxury meat and fish, like steak, prawns, once even scallops, all for the ubiquitous £1. The freezer section of her fridge was stacked with bargains to be eaten later.