These may be obvious to others but have really made a dent chez Hummus:
Freecycle - tons of local groups with people who'll collect unwanted furniture/clothes/toys/whatever from you free, or who advertise unwanted things. I once saw an old Toyota on there FFS - you could basically kit out your house from Freecycle in a pinch.
The freezer - we tend to buy meat and frozen veg, and just defrost/use as needed.
Bulk cooking - I'll make stews/bolognese type dishes in 3x quantities, freezing most for later use. Also, that MN stereotype of adding lentils to everything really does work.
One-pot dishes - stews, soups, dishes where you just chuck in loads of ingredients and leave to slow cook.
Lidl - Just as good as Waitrose for most things, far cheaper.
Online shopping - no temptation to pick up random things, much easier to stick to your shopping list.
Planning for 5/6 days worth of meals, not 7 a week: there's inevitably one day a week where we'll fancy a takeaway / one of us will eat late in the office / we'll be out for a meal with friends.