These are the things I have learned in my years of rehearsing and gigging with several bands. As I am a bass player, some of these tips are from that perspective:
Music Stand: This is for those of us who have set lists and key structures written out. The stand is better than picking papers off of the floor that were on your amp.
Instrument stand: I can’t even count the times that my guitarist kept forgetting his. He’d prop up the guitar and I would watch it fall across the amp. Oh well. Save your own instrument.
Ear plugs, for the obvious reasons: loud drummer, super loud guitars, etc.
At the gig: Don’t drink excessively or dope up. It shows in your playing and your professionalism or lack of.
Practice: Send a flier to the neighbors letting them know when you practice, so if there is a problem, they don’t call the cops first, they call you.
Band politics: The band sits at a round table. There are no egos, and everyone gets a say. Be democratic. Discuss your goals, tunings, dynamics, etc.
Towel: To sop up the spilled beer on your amp that your guitarist denies he caused.
Cable: Loop your cable through your amp handle and your strap so it doesn’t fall out.
Picks: Different gauges for anyone in your band who uses one.
Spare cords and cables: It worked yesterday, so why doesn’t it work today? By the way, wrap your cords when you are done; the copper wire has a memory.
Spare strap: I had a guitar player sit in a chair for the whole gig. Enough said.
Spare microphones: If you sing, it is always good to have a back up. There is nothing like watching, in slow motion, a microphone hitting the ground and splitting into pieces.
Tuner: A guaranteed need. Your guitarist thinks he’s in tune and everybody else is out.
Batteries: For the tuner (which proves your guitarist wrong) or effects pedals.
Strings: Obviously for yourself. But also get a set of guitar strings. A guitar string is guaranteed to break; it’s always the high E. This happens on a Sunday, Sam Ash is closed and the gig is in an hour.
Drum key: The drummer arrives and can’t tune his set, or can’t put a new skin on.
Power strip/surge protector: For some reason, clubs always have problems with their power. Go figure. There have been more surges or blown circuit breakers than I care to remember.
Screwdriver: To get into your instrument or amp if there are any problems.
Allen wrenches: A must, if you want to do any instrument adjustments.
Tubes: For all of you oldies out there.
Fuses: For GK amps especially.
Pen/paper: For some reason you always need this. I just don’t know why.
Crazy glue: For your fingers or your guitarist’s mouth.
Duct tape: Repairs anything — even your guitarist. It can even fix broken drum skins.
Duffel bag: To carry everything.
Spare Clothes: For yourself. Not the kinky kind, but something that is comfortable, or to change into if someone spills something on you.
I know you will feel like a walking hardware store with all this stuff. But don’t tell your band mates what you have. If you do, they will rely on you every time. The purpose of having all of this equipment on hand is to save the day for yourself and others. Most importantly, it brings a sense of professionalism to your craft.