I'm sure some bassists already know this, but if you can't afford buying new strings often, take them off and boil them in ether (do this OUTSIDE and be careful, LONG extension cord, plug it in when it's far away, let boil, turn it off, let cool at least 30 min. before retrieving them)
Enjoy your old set of strings sounding like you just put a new set on.
Years back when I played with a perpetually broke jazz band the bassist used to do this once a week.
If you blow yourself up doing this you didn't hear this from me (Jedi wave...)