Make sure that all the details for your show are hammered out. You will want to speak with whoever booked your show (the promoter, booking agent or club manager) prior to arriving at the club. Never just assume that everything is going to be fine. Always check. Make sure you have the following issues resolved before heading out to your gig: time of set, load-in of gear, sound check, payment details, parking, if the club provides a backline or if you'll have to bring your own amplifiers.
If you are not a solo musician but in a band, you will need to have one person in charge of rounding up the troops. It does not have to be someone in the band. This can be a mutual friend or family member, as long as they are dependable and organized. This person must have a list of all band members with complete contact info for each person. The day of the show, call everyone and inform them of the central meeting time and location. Try to limit any extraneous information because most musicians are highly excitable and easily distracted. Keep them focused on their role.
After that, you will want to arrive at the club at least 30 minutes before your load-in time. This is because musicians are generally late and if you tell them to arrive 30 minutes early, they will get there right on time. Another factor is forgetting equipment or merchandise at home, which always seems to happen even if you double-check before you leave the house.
Now you need to keep the band members from getting too drunk before they play. Just get a few girls to talk to them before they go on -- this will keep them occupied and away from the bar.
When the band before you finishes up, please be courteous and allow them a chance to clear out from the stage. Don't try to haul all of your equipment up there before they are ready. This will spread bad karma before your set and will actually get them off the stage much slower. Just be patient.
When loading your gear on stage, make sure that nothing on the stage gets damaged and that your equipment does not get damaged as well. This is when your big-bodied friends come in handy. Some of those speaker cabinets can weigh over 90 pounds a piece. So do not hurt yourself before the show -- get by with a little help from your friends.
Play a great set. Connect with your audience and be expressive. This is the moment you have worked for. Make eye contact with the patrons in the bar and do not turn your back to them on stage. Focus your energy and they will reciprocate.
After the show, try to be polite to the band performing after you (assuming you are not the headliner). Get your gear safely and quickly to your vehicle so you can get back in the club and say hello to everyone inside. Don't forget to lock your car -- stolen gear will ruin the night.
The final and most important action is "settling" with the promoter at the end of the night. This is when you will want your big-bodied friends nearby again. It is not uncommon for the club manager or night promoter to use questionable accounting methods when determining what you have earned. Get an accurate head count and do your math before you approach the promoter. Have a friend keep a tally of people who came to see you perform. This will help out if a dispute arises. If you enjoyed working with the venue or promoter, ask when the next gig is available.