Bring spares, for everything. Even the spares.
Bring enough strings to last four times as long as you are going to be on the road, and string winders and clipers as well. If you have a locking trem, do NOT forget to bring a few spare sets of allen wrenches. Bring spare switches, pots, caps, jacks and wire for your guitars. A couple of extra pickups are not a bad idea either, nor are spare gears, and spare saddles for you electric guitars are good too. And of course the nessicary screwdrivers, soldering irons, pliers, etc.
And bring some guitar polish. There is no reason to go out on stage with your guitar looking dull.
A couple of spare sets of tubes is a MUST have, and a extra speaker or two are not a bad idea. If it were me, I'd have a spare head as well, though I don't know that I would have a spare cabinet as they are easy to fix (provided you have the stuff to do it).
And for gods sake, don't for get to bring a box or fuses.
Don't forget to bring batteries. Yes, you can usually buy them on the road, but then again if you have any Sunday night gigs... just don't risk it.
You should have enough cables to rewire your whole rig at least twice over.
If you play slide, bring three (spares for your spares, right?). Capos as well.
Of course, every piece of your rig that you absolutly can not do without, you need to carry a spare. (Need to have that delay for your cover of some U2 song? Better bring two.)
Being me, and having built every piece of my rig, I would also be bringing spare transformers for my amps, along with spare tube sockets, resistors, capacitors, etc. for my amps and tools for doing fret work, truss rod tools, a straight edge, etc. for my guitars.
Bring lots of clean underware, and laudry detergent (for those days you actually can find a laudrymat).
But MOST IMPORTANTLY, bring a multimeter and an outlet tester, AND USE THEM EVERY DAY!!!!!!!!! Check to make sure the wall outlets are not wired wrong before you every plug in a single piece of gear. Check the voltage between hot and neutral (it should be about 120v), but also between hot and ground (also 120v), and neutral and ground (0v). Make sure that you are not pluging your guitar into two different circuits, and that there is no voltage between your mic (use your meter set to check for AC and DC voltage, touch one end to your strings, and the other end to the mic.)
This can save your life, literally.
Oh yeah, and don't forget to bring plenty of condoms.
Have fun!
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M.K. Gandhi