The two categories that actually move

Almost everybody finds the same two are elastic: food, and going out. Rent is not negotiable mid-term and neither is a course fee, so those are settings rather than decisions. Concentrating effort on the two that move is where the effort pays.

For food, the mechanism is cooking a small number of things well and shopping once with a list — because the expensive pattern is not restaurants, it is buying the same day you eat, tired, at the nearest shop. For going out, decide the amount in advance rather than each time, so the decision gets made once while it is still cheap to make.