Besides day to day cleaning etc I recommend getting a cable tester, a multi-meter, a soldering iron, some reels of cable (balanced, unbalanced and speaker) and spares for everything you can fix yourself (eg. a spare set of tubes for every amp, fuses, jack sockets, connectors, pots, caps etc). It's much quicker to put a cable together yourself/throw in a new set of power tubes than to rush out to a store to buy one or wait days for a tech to sort out an amp.
If something breaks you can't fix I always have a list of local repair and hire companies so I can replace it (whilst getting it fixed) within hours if that's what's needed.
I check over all my main equipment live and studio before any session or tour and eliminate any possible issues beforehand, I've yet to have a major issue mid session/gig with my gear but I don't want to not be prepared if I do.