This is a simple post, what do you need with you to survive out in the world of 'Guitar Land'?
However - I am NOT going to tell what you need.
"WTF Trashed, I thought you were trying to help us?"
"Well... I don't know what you need because I am not you, but what I WILL DO is tell you what I bring".
I am assuming that you have two guitars, and a tube amp that you are taking, although a backup amp is always good to have access to.
I have a tool bag that I got from Home Depot like this: http://www.homedepot.com/p/Husky-12-in-Tool-Bag-82004N11/203224030. $10 and rugged as all hell. I own at least five of them for different things. They go cheap for Black Friday too, I think I got mine at $4 each last year.
There is one thing that separates a pro player from a new player: A beginner will bring enough in case ANYTHING were to go wrong. A pro will bring enough in case EVERYTHING goes wrong.
That is it right there, take that as a word of wisdom.
I am not going to list the common things (picks/strings/etc). I am going to cover some of the lesser thought items, some are obvious, some a little less so. This is a combination of general, guitar related, and amp related.
1. Towels/rags - you want to be able to wipe down your guitar strings, body, fretboard, forehead. It gets hot up there.
2. Magic marker/sharpie, pen and paper -to get the bitches numbers right? Yeah, but ya' know there are other reasons. Set lists/marking knobs on amp/pedal. etc.
3. Flashlights - one of the most important actually. It can get dark on stage, and if you bump a knob, you need to see it. If you need to plug/unplug something during the set. If you drop something. etc. good for setup too.
4. Batteries - for EVERYTHING. 9V's for pedals and if you have active pickups, flashlights, wireless systems (if applicable), and even for your clip on tuner.
5. Fuses - usually there are two fuses on each amp that are accessible from the back panel, make sure you have the SAME value, and the SAME type (standard versus slo-blow).
5. Cables - that falls on the obvious side of things, but this is something that can completely impair your gig if one goes. Cable testers are a good idea overall too.
6. Extension cords and power strips - . Everything needs to plug in, and outlets are not always plentiful or not even in close proximity. I personally use a power conditioner, but that is a luxury to some degree.
7. Tubes - Here is another obvious one, wrap them in towels, have some of every kind your amp uses.
8. Extra power supplies - a 1spot is always good to have in case your iso-brick were to die. Or if you are using a power supply, bring a second.
9. Allan wrenches, screw drivers, a telescoping magnet on a stick. Tools in general.
10. Tooth picks and chopsticks - you can safely poke around your amp with chopsticks if something were to go wrong from the 'tapping test' on tubes, to if you drop something in there. Toothpicks are handy in case a strap button were to fall out (yes it happens), insert and screw the screw back in. Temporary fix.
11. Digital multi-meter - good for everything, especially continuity (set on ohms setting) which can quickly troubleshoot a dodgy cable or connection or short.
12. Second tuner. Period. It doesn't matter what you have, get an extra. If you can't tune, to a degree you are screwed.
I am assuming that you have a backup guitar and access to a second amplification source. If you don't have a second amp to bring, sometimes other bands will let you use yours in a pinch. It is always good to have something small that could go into the PA if needed, From a POD to an Orange Micro Terror. Personally I keep a Peavey Valve King in the trunk. I haven't needed to break it out yet.