Long flights.
Plan trip, note still air time, mark on flight plan in big letters fuel tank change over point. Select alternates, particularly en route alternates.Assess wind influence. Fill to the brim. Try to keep hand on tank change knobs 20 mins before empty time, and change tanks 10 minutes before theoretical dry time, earlier if course or frequency changes are coming up, or if weather is iffy.
Aim to land with a theoretical hour of fuel in the remaining tanks.
Short flights, one pair of tanks full, and at least 2.5 hours fuel for 1 hour flight.