If you play music in public with any regularity, especially on a local or regional level, you will quickly become part of a community. As you play more shows, or attend other artists' shows, you'll meet other musicians, sound guys, groupies, club owners, managers, etc., and you'll be surprised to discover that they also know so-and-so, and what's-his-name.
A lot of the musical successes you experience will be as a result of help you get from these people. Maybe a band you met 6 months ago has to bail on a gig and asks you to fill in for them, and you end up selling tons of merch. On top of that, the club owner books you for a future date because she likes your sound. Or maybe a solo artist heard your band at an open mic, and asks you to play drums on his upcoming album.
Because music scenes are so small and so social, it's a good idea to try not to piss off other people in the community. I know this only because I have pissed off or have been pissed off by many people in my musical experience. Learn from my mistakes...
For me at least, this is harder than it should be. Time your set during a rehearsal. Make sure you have easy access to some sort of timepiece when you're on stage. Write approximate song lengths on your setlist so that you can quickly shuffle things around if needed.
My band once played a show where we accidentally went over time because a mic crapped out mid-set and the soundguy had to scurry around to find a replacement. When we got off stage, the show-runner was clearly pissed, and told my bandmate that he "didn't think we were THAT kind of band." We didn't get asked to play with those guys again.
Artists take their sets seriously - they spend a lot of time preparing for them, and expend a lot of energy trying to get people to show up. Treat that time like it's money, and don't steal from other bands.
You just got done playing your set, and there's 2 other bands playing after you. If you're tempted to just load up your gear and go, try and remember that those bands probably had to sit through your set, and it's nice to return the favor by being part of their audience, especially if the show has low attendance.
Even if you aren't into the other band's music, try your best to look like you're enjoying yourself. It'll make the other band feel better, and make the set go that much more smoothly. No artist wants to look out from the stage and see crowd members, let alone musicians, crossing their arms and scowling.
If you end up playing with that band again, the performance order might be switched and it could be them who's packing up their gear and heading for the exit while you're playing. However, if you stay for them this time, they might stay for you next time.
Even worse than leaving before someone's set is leaving DURING their set.
Even worse than leaving before someone else's set is leaving DURING their set. Watching people leave while you're playing music is a big confidence killer. You don't like it when it happens to you, so try your best to avoid doing it to someone else. If you HAVE to leave early, make sure you apologize to the band before they start, and make it up to them by coming out to another show in the future.
My band played a festival a little while ago. The stage area was under a small tent, which we shared with the sound guy. We were in the middle of our first set when the next band (let's say they're called the "Delaney Family" - not their real name) arrived. They pulled up a couple of folding chairs next to the sound guy, and started a conversation with him. They continued to converse throughout the remainder of both sets. I could hear a good deal of their chat. Some of their discussion could probably be heard in the mic.
I'm sure the other band didn't mean to be distracting, and it certainly isn't the worst thing they could have done, but my clinically-diagnosed case of Adult ADHD means that I was processing their conversation while trying to remember lyrics, chords, and stops, etc. Aside from that, the fact that they were talking to the sound guy means that he wasn't paying attention to... the sound. All in all, it's kind of rude, and it's easily avoidable. Be considerate, and take the conversation somewhere away from the stage.
Back to that festival I mentioned. The "Delaney Family" was very excited to play their set after we were finished. I know this because I had barely gotten the words "thank you everyone" into the mic before the "Family" members were on the stage adjusting mics, pulling guitars out of their cases, and flipping through sheet music. My band had to work around them to get our gear packed up.
Wait for the other guys to clear their gear before you hit the stage, and clear your gear as quickly as possible.
There was plenty of time in between sets to allow us to pack up, and to allow the Delaneys to get ready to start. There was no reason to clutter up the stage area and get in each other's way. Unless you get specific instructions from the show-runner to set up while someone else is tearing down, wait until the other guys are fully clear of the stage and all of their gear is packed up.
On the other side of things, make sure you get off the stage as quickly as possible to give the next band the most time to set up and troubleshoot any issues. Don't go running into the crowd to hit on that chick at the bar until everyone in your band has cleared their gear off of the stage.
This should be common sense as far as human decency goes. If you see someone carrying a lot of gear, or if it's heavy gear, help them. Just don't drop any of their stuff. If you're a musician, you know how back-breaking some gear can be, so help a brother/sister out when you can. At the very least, open doors for them as they carry their stuff. But if you do drop something, run.
This seems to be common knowledge, but just in case it's not... At some point during your set, chat up the other band(s). You just HAVE to make sure to get their name right. Even if you shout to the band from the mic to get their name right (I've done this many times), you're basically just doing the other band the favor of repeating their name in a positive light to the audience, subconsciously reinforcing the idea that, "hey those guys are cool, and everyone likes them, and you should buy their stuff."
You're also endearing yourself to the other band, which could work in your favor in the future. What musician doesn't like hearing someone say how good they were? You're ALSO creating a shared experience with the audience - "hey, remember when we all heard those guys play? That was awesome right?" It's a small way to build a connection with the crowd.
Talking negatively about someone else makes YOU look like a jerk, especially if you're talking about them when they're not in your presence. Music communities tend to be somewhat small, so word travels fast, and if you become known as a behind-the-back-talker-guy, you will damage relationships, which could end up costing you, even if you aren't aware of it.
As an example, even if you couldn't stand the other band's music, trust me, it's best to be diplomatic and pretend that you did. Believe it or not, the world isn't waiting on your opinion... you can keep it to yourself. You want to be viewed as a Pro by the other people in the music community, and Pro's don't talk smack.
The only time you should ever use someone else's gear is if you have express (preferably written) permission in advance of the show. You need to give the gear lender time to decide if he actually wants to lend without the pressure of you standing in front of him, eagerly awaiting his answer.
On a couple of occasions, show-runners have told me that they already talked to the other band, and the band said it was okay to use their stuff. That doesn't count as permission. Make DIRECT contact with the gear owner and get express permission. You never know what details could have been lost, misunderstood, or forgotten in the communication between the show-runner and the other band. If you show up to the gig and the band is either not cool with you sharing their stuff, or they have to leave before your set, then what are you going to do?
If you're sharing gear, you're placing a lot of trust in a stranger.
If you're sharing gear, you're placing a lot of trust in people you may not know very well. What if they forget the gear? What if they get sick and have to bail out of the show just before you go on?
If you do end up sharing gear with another band, try not to change any of their settings. However, if you absolutely HAVE TO mess with their settings (after getting the owner's permission), take a picture of the setup before you touch anything so that you can get it back to where it was after you're done with it.
Pressed albums can serve as one form of business card. Exchange them with the other bands, or if you don't have your own, seriously consider buying the other guy's. It will earn you some good will. Take the album home, listen to it, and try to find something you like about it, even if you aren't really into it. Reach out to the other band and let them know what you liked. It's an easy excuse to stay in contact.
Find out where the band is active online, whether it's Facebook, Twitter, their website, or whatever site the kids use these days. Reach out as soon as you're able, and keep in touch. Comment on their songs. Check out their show schedule, and go to their shows. Stay on their radar, and they may ask you to do a show with them in the future. Your presence on their social media could also earn you some new fans.
The takeaway from all of this is: being a good musician isn't just about playing an instrument well. It's also about being easy to work with. People who are easy to work with get more work. Jerks get ignored. Even if there are no clear benefits to you, being a friendly, supportive musician to other musicians makes a better music community, and I think we all want that.