How to not forget anything?
What could be more exasperating than playing without your favorite distortion pedal simply because you forgot a 9V battery? The best way to avoid such inconveniences is to have a checklist and review it before you leave for the gig, like a pilot before takeoff.
Here you have a printable list that you can customize as needed.
It can be useful before but also after the gig, so you don't forget anything at the venue.
If you aren't fortunate enough to own two guitars, but you have an altruistic friend, you can ask him to lend you his guitar. Because it's not an easy task to change strings on stage with all the stress, the sweat and an impatient crowd in front of you.
When to change strings
I'll say it loud and clear: There's no magic solution that applies to all guitarists.
A seasoned musician who is used to doing it by the book can afford to change strings just before soundcheck without the risk of ending up being dangerously out of tune. But for a beginner who is less experienced, it's better to change them at least one day before, in order to avoid having to tune the guitar after every song during the show.
In any case, changing the strings won't exempt you from taking along a new set of strings. String wear plays only a minor role in the breaking of a string (your playing technique, the thickness of the pick and luck are much more important factors).
Do your homework
Band rehearsals are crucial, but that's by no means enough. Practicing at home is the best way for your fingers to learn by heart those complicated movements. However, you risk losing your focus easily if you practice lying down on your bed.
You need to practice under proper conditions, which implies at least standing up with the cable around your legs. If you have any recordings from rehearsals (or any other practicing media), don't hesitate to play along with them, cranking up the volume enough to mask the sound of your guitar. Why? Because you surely aren't expecting to hear yourself perfectly clear during the gig (especially if you play an acoustic-electric guitar), are you? So, what you want to achieve is that your fingers can follow along mechanically without losing a beat during moments of panic.
Finally, don't pay more attention than needed to the quality of your playing. In a small venue, with an often unpredictable sound, the crowd will focus more on the energy you put out than on the E flat you missed during your solo.
So, enjoy it and the crowd will go along!