Whatever can go wrong will go wrong. It's an immutable law of nature. This is especially true of the concert stage, where egos, beer, bright lights, loud amps and winking groupies can wreak all sorts of havoc. But you can minimize the impact on your show by thinking ahead and being prepared. It would be great to pack your entire stash of gear when heading out for a gig, but that's not terribly practical, especially if you suffer Gear Acquisition Syndrome (GAS), so every guitarist needs a gig bag packed with emergency supplies. Here are some live stage lifesavers.
Busting a string on stage can be an utter gig-killer. You can minimize the harm by bringing along extra guitars if you have them, but if you don't you'll need to become a string change ninja. But don't just bring extra strings: bring everything you'll need to do a restring quickly. That means a string winder and something to cut the string ends with. I know, I know, leaving the ends on looks kinda rock, but in the heat of passion during a gig those little loose ends become lethal weapons, and then you've got a lawsuit on your hands. And here's a handy hint: If your guitar has a Floyd Rose bridge, prepare a spare string set before you go to the gig by snipping off the ball ends. Also, use a tuner that has a mute function so you don't bore your audience with tuning sounds. Please.
You can never find a pick when you need one around the house, so imagine how much it would suck to not be able to find one on stage. Remember the Tenacious D episode where Jack Black lost his favorite pick and wouldn't continue the show until he found it? Don't be that guy. Bring along way more picks than you'll ever need, and stash them everywhere. On top of your amp, a couple on your pedalboard or in front of your mic stand (or on the mic stand if you have one of those cool pick holder attachments that guys like Yngwie use), a few in your pocket, and maybe one of two over on the bass player's amp just in case you find yourself over there. Some players even stash a few on the guitar itself.
Speaking of picks, here's an important tip courtesy of a recent Marty Friedman clinic. Marty prefers to not use one particular type of pick. He uses a whole range of different ones - a plectra spectra, if you will - partly because he likes how he plays differently with different picks and partly so if he's ever caught without a pick he can just borrow one from someone and not feel lost. That's a pretty valuable lesson for the gigging guitarist. Don't ever become so reliant on one piece of gear that you can't function without it.
Fuses and tubes
“No user serviceable parts inside.” We've all seen it written the back of our amp. That doesn't mean you can't change your own fuses. Know what fuse your amp requires, and bring a few of them. Changing fuses is not rocket science - well, I'm sure rockets have fuses somewhere but you know what I mean. As for tubes, although some players like to carry a spare set just in case, a gig isn't really the optimal place to perform a tube change. It's dark, there's probably a lot of beer around, and there's definitely lethal voltage lurking in your amp. If you can, avoid a tube change by bringing a backup. Which leads us to...
Backup for amp
We don't all have the luxury of lugging along an entire second amp just in case the first toasts a tube or fries a fuse, but with a little forethought and a small investment you can at least have something that will keep the gig rolling if your amp goes down during a show. If you don't already have a digital multi effect pedal unit, get one - make sure it has speaker emulation and will play nicely with a mixing desk - and create a few patches that mimic your amp channels. Test it out through a PA system during a rehearsal session if you can, so you can iron out any kinks and so you know exactly how to set it up should the moment arrive. Another alternative is to use amp modeling software on your computer, iPhone, iPad or iPod Touch if you have one. Again, if you go down this road, test it out in a rehearsal setting at least once just so you know the score.
Hex keys (Allen Wrenches), screwdrivers, wire cutters, gaffer tape, torch. Heck, if you can manage a soldering iron too for absolute-emergency repairs, do it! War story time: during a gig supporting the band The Screaming Jets, I overshot a bend on the B string and yanked the Floyd Rose bridge saddle right past the base and onto the top of the guitar because an intonation screw was loose. Upon releasing the bend, the string was tuned somewhere around G. Whoops. If I didn't have my hex keys I wouldn't have been able to fix the problem (by jamming the saddle back to approximately where it was and locking it down - you don't have time for a full Floyd Rose intonation session when you're in the middle of a gig).
For every cable in your rig, make sure you have a replacement. This can get pretty costly if you use a multi effects processor and amp wired up in the four cable method (where the amp's preamp is effectively placed into the loop of the effects unit for greater flexibility), but the cost of a few cords pales in comparison to the shame and humiliation you'll feel if you have to play “Voodoo Child” without a wah wah because you're one cable down. Also make sure you have replacement speaker and power cables for your amp just in case.
Tell yourself to put fresh batteries in your pedals (and guitar if it needs them) on the day of the gig. You'll forget and your rig will go silent at the worst possible moment, but that's why you also have a stash of batteries in your gig bag too. Next time you will put fresh batteries in before the gig and this won't happen at all, but at least you've saved this gig.
Business cards or CDs
You never know who you're going to run into at a gig. Put on your networking face and make sure you have CDs and/or business cards at the ready. In fact, business cards are probably a better option in a lot of cases because the last thing you want to do is saddle an important music business luminary with a CD that he or she then has to carry around for the rest of the evening. Make a business card that features a URL to download your demo instead.