What do you do to combat nerves before a performance?......if anything
I've fortunately not been too prone to being nervous on stage, even from the earliest gigs. I think it comes from not performing with the thought that it's about me, meaning, I'm only one person in the whole band, so I can't be thinking that everyone is focused on me, nor are they just waiting for me to make a mistake. So I guess I sort of hide in the band collective for security.
Secondly, I found that 99% of the time, most of the people coming are there because they like you or are interested in the music you do, (if they don't know the band), or are just out for a good time with friends - so it's only the real fans, that are there because they like you, that I have in mind when I play. -- Once I was heckled with a guy yelling "GET OFF THE STAGE!!!" - I was initially mortified, until I realized that he was 1 person and everyone else was applauding and enjoying the show. Heck, he could've even been a friend yanking my chain, for all I know - but that one event taught me to not take myself or the crowd too seriously, and I learned to light-heartedly heckle back in fun. So, I play to the people that are there because they like it. You can't please everyone, so why worry about it - that's normal.
Thirdly, I do most of my nervous stuff BEFORE even leaving for the gig. I found that a lot of nervousness comes from preperation, or the lack thereof; and it's usually in 2 areas : a) personal practice, being ready and knowing your parts well. This amounts to wood shed, wood shed, wood shed... and DON'T gloss over your weaknesses. Play through material WITHOUT any cheat sheets or crutches, like playing along with recordings or only rehearsing with other players where you depend on them for cues. Practice your parts by yourself and when you hit a snag, give that snag any special attention that it needs in order to OWN it. I do that on every song, it's the only way where I know my weaknesses so I can focus on, and conquer them, and not waste time on the easy stuff I already know. And DON'T be cramming the night or day of before the gig, start as soon as you know the date to fix weak spots.
And b) Gear prep + gig details. I insist on knowing all the gig details well in advance - days, at least - venue location, load in time, doors open, sound checks, gear sharing, other bands - anything day of show that will help me not not be surprised when I walk in the door to set up. That's a real stress reliever for me. Also, at home, I'll pack my gear, after making sure that I have everything in working order, and get it all ready to load - most often - the night or day before, except for my bass and a practice amp for a little warm up to retouch on those iffy parts that I've already been paying attention to. IF I need it, on my set list, I scribble some tiny notes, like key, starting riff, or an elusive chord progression, just in case, so I can do a quick glance if I need it.
Also - never use new gear that hasn't been rehearsed and well tested at a gig - quite often, it'll trip you up. The same goes for using an untested chart. I avoid using charts now, but I'd previously throw together a sloppy one when learning a tune, then use it exclusively until gig time. I used to rewrite them the night before very neatly, which was often disorienting at the gig, since I was so use to the sloppy one. - So my rule now is, either do it neatly right away or stick with the chart you know. But better to not use a chart at all if you can avoid it. Your brain well rehearsed is much better at recovering from mistakes than fumbling to refind your place on an unfamiliar chart.
Otherwise, like others have mentioned - Mistakes? fehgetaboutit ! You can't stop and fix 'em, so move on and forget them. Don't make faces when you do them to call attention to them or yourself. If you must react, LAUGH. It looks like you're having fun. And truth be told 99% of people don't even notice, even if it seems like a huge clam to you. It's usually your own head that does one in.
A friend of mine has the habit of worrying that he's gonna screw up before even playing the song - thus, a self-fulfilling prophecy comes to pass. To make things worse, after the mistake(s) he dwells on them through the rest of the gig, while looking at other people, imagining that they think he sucks - when nobody cares or is the wiser. He's been so self conscious that, if I make a mistake and laugh about it, he's thought I was reacting to his errors. Which circles me back to my initial "It's not about you".
Music is best, in my opinion, when it's LIVE and on the edge, played with lightheartedness and a slightly confident (but not egotistical) attitude. Put on your best "rock star in front of the mirror at home as a teenager" persona when you hit the stage. Take on a character outside of yourself, put on your stage performance persona. It can really help to "be someone else" - like you're acting a part.
One friend told me a quote from somebody "When I go on stage, this is my turf, I own this space while I'm here, nobody else matters for that time".
Which brings me to LASTLY: When I'm on stage, I don't usually play to the crowd, I play to my band mates, that's the space I'm in, big stage or small, regardless of audience size. I can't fully connect with the whole crowd. But when I have, I treat them all like one person, not seeing numbers. They're a collective feeding off the energy of the music and each other, so it's often best for me to focus on the music in context of the band, first and foremost. All that matters is that we perform our best together. The crowd is along for the ride.
I had a student - a solo pianist - who was going into a competition, and she was terrified. I asked her, where is the space that you feel the most happy and comfortable when you play your music. She said, in her room at home, alone with her piano. -- I told her, when you take that stage, and sit at the bench - Stop. Shut your eyes. And see yourself at home and tune into that feeling for a moment. Then hold onto that, and from there, begin to play, just you and your piano. She did that, and she won.

Groovy - and even advanced to regional state competition. -- She did her homework, knew her music, she just had to find, as they say in Peter Pan, her happy place. And even though it was at home alone with her piano, she always carried the vibe of it inside her everywhere she went.
Hope all the wisdom of the forum helps you.
Oh, and little to no caffeine or things to make you jittery - like energy shots, etc. - get good sleep pre-show, take your vitamins, forget about the gig and the crowd and go have fun collectively failing and triumphing with your mates. What could be more fun? You get to be a rock star!
