you can get a little more mileage out of old strings by detuning them so they’re super slack, then repeatedly plucking them hard and letting them snap back against the fretboards (as if you’re (slapping and) popping a bass string). This knocks a whole load of rust, dirt and oil out of the strings and brightens them up again!
i'll go months, even years, without changing the strings on my guitars, until my strings start snapping. yet every single time i change them i'm shocked by how much better i sound as though its magic (its not)