Paying interest on debt whilst you save.
Before you can start to save, it makes sense to clear your debt first. It makes no sense to be paying high interest on a loan or credit card, while earning low interest on savings in the bank.
Turning on the heater without thought!
My Husband is awful for this. He'll be sat at home wearing shorts and a t-shirt and turn the heat pump on to warm him up. Layer up the clothes with jumpers, jackets and slippers before adding to your power bill. Also think about heat losses through bad insulation. Bubble wrap is a great insulator for windows.
Premium brands.
Don't be afraid to give Pams, Budget and Homebrand a try. They can often be better tasting than premium brands and cost a fraction of the price. Try Countdown's tomato paste for a grand example! Knocks socks of the more expensive brands and freezes well too!