Switching off the lights when they are not in use will not save you much money. But when many people change their habits they save not only their electricity bill, but also resources used by power plants to produce electricity. For example, when you go to the kitchen to cook/have dinner for 2 hours and don't forget to switch off the lights in the living room/cabinet/children's room, where you have efficient lamps with a total capacity of 135 W (e.g. nine lamps, 3 in each room), you will save about 99 kWh/year (15 W * 9 lamps * 2 hours * 365 days).
No investments, just creating an electricity saving habit.