One area that is often overlooked is that of tools to always have in your tool kit at every show. You’ll need tools to test, and tools to fix.
You’ll need a fairly good multimeter. With this you can test the AC line voltage. If you seem to have lousy headroom, and some of your switching doesn’t work well, check the line voltage. It should be somewhere in the 115V-125V range. You usually won’t have a problem with the voltage being too high, but you will sometimes run into voltage that is on the low side, and it can affect how much headroom your power amps have. In drastic cases, it can cause distortion at nearly any volume.
You can also use it to test cables, guitar pickups, and components of gear when you have to pop the hood on something.
Also, go to Radio Shack and buy a $6 “Line Tester.” This is a real life saver (possibly literally!). It checks the outlet to make sure that none of the conductors are switched. I use these on every outlet I will have to use before I plug a single thing into them. The most common flaw they will find, which is difficult to check without one of these, is that of the neutral and ground being switched. If you try to use an outlet wired like this, you will end up with hum at the very least.
A 9V battery is a requirement. A quick, easy, and reliable test for speakers. Don’t leave it connected for long, and never use it on a compression driver! Use a jumper to connect the negative connection to the (-) side of your speaker, and briefly touch the positive side to the (+) terminal on the speaker. The cone should jump forcibly OUT. You should hear a ‘thump,’ but no scraping or rattling. Connect it to the speaker cable leading to your speaker, and all of the speaker cones should move the same direction. This is VERY important. If any of them move in while the others move out, then the one that moves in is wired out of phase. Fix it right now. It’s killing your low end.
You can also use it as a ‘down and dirty’ way to quickly check cables when you can’t get both ends close together (such as a snake you’ve already laid out). Connect the battery (-) to Pin 1 and (+) to Pin 3. Grab your multimeter and go to the other end of the cable, put your meter on a DC Voltage setting that will cover 9VDC, and you should find +9V on Pin 3 (referenced to Pin 1) and nothing on Pin 2. Connect the (+) terminal of the battery to Pin 2, go back and re-test with the meter. You should now have +9V on Pin 2 (referenced to Pin 1). You’ve just completed a very basic test of that snake channel. You haven’t tested everything that can go wrong with it, but it should pass sound. If you don’t get a reading on either Pin 2 or Pin 3 with either test, then there is a problem with the Pin 1 connection (the shield). If one gives a reading, but not the other, then the one with no reading has a problem. The problem is usually at the mixer end (the pigtail). Pull that XLR shell and examine the connection.
A soldering “iron.” Actually, I would carry two. One fairly hefty one to repair connections on larger cables, patch bays, etc. And one smaller one for mic cables and for fixing the insides of your equipment. Also, be aware that if you are doing an outdoor show when it’s chilly and windy, a smaller soldering iron may NEVER get your connection hot enough to make a good solder joint.
Solder wick, and good solder. (Don’t forget to tin your wires before soldering!!)
Pliers, needlenose pliers, vise-grips, wire cutters, a good razorblade, electrical tape, a MiniMag, or other flashlight small enough to hold in your mouth so you have both hands free.
A good selection of screwdrivers. As basic as this is, you wouldn’t believe how many people don’t carry even a basic set. You’ll need a tiny flathead for those screws in XLR shells. You’ll need a few good Phillips (or whatever matches the fasteners you used to mount all of your rack gear). Wrenches and whatever you need to take the back off of a cabinet and replace a speaker.