This will be unpopular. Maintenance. Of everything: loose jack on the instrument, loose tuners, loose strap buttons, batteries, cables, cords, clean up, inventory of strings, straps, picks, tuners, FX box maintenance if you are into that, check all the bridge saddles and nuts for wear, burrs, knobs on the amp, DI box integrity, tubes if you're into that, etc. All that gets expensive really, really, quick, especially if it fails during a gig. Just like a car, everybody wants to talk about a new car, or new accessories, but nobody wants to talk about the cost of proper maintenance to keep it looking and driving new. Same way with a gigging bassist. For example, in the middle of the last gig, the EXB control on my bass crapped out. So instead of anything else, the first thing I had to do was not only purchase a replacement, but check all the rest of the gear to do what I could to not have another failure in the middle of a gig. I found a couple of frayed cords that, although they did not contribute to the failure (internal pot wafer crumbled) could have at a subsequent gig. And so on. I have seen too many guitar players that lost a pedal, lost an acoustic guitar pickup, string breakage, etc., all that could have been avoided by good pre-flight check routines. Yes, number one when a person receives a small gift or windfall like that, or the share from the tip jar, is maintenance. No, afterwards there doesn't seem to be anything to show for it but the confidence at the next gig, which is huge. Did I say maintenance?