Plenty of preparation goes into your typical DJ set: purchasing and sorting new tunes, selecting tracks for the crate, and of course hours and hours of practice. Once you get to the DJ booth, there's nothing more exciting than taking control of the system and sharing your sound at high volume'"but there's a few typically overlooked considerations to ensure everything goes smoothly at the gig.
We live in a chaotic universe and things all too often go wrong when we least expect it; Murphy's Law holds especially true in dark, loud venues frequented by lovable degenerates, and every DJ booth comes with a set of relatively predictable variables. That being the case, there are a number of strategies to minimize your risk of mishap, regardless what medium you use to play.
Spinning vinyl? Bring an extra set of needles and cartridges; you can even find inflatable shock absorbers to bring in case of skipping needles. Using USB-enabled CDJs? Always bring at least one extra USB-stick, as well as your own sufficiently long ethernet cable in case they're not networked for some reason. Using a digital vinyl or controller-based system? Always have extra audio and USB cables at the ready in case one decides to crap out on you.
No matter what format you're playing, bring music in another format as well: at least a few 12' records if you can, and always a few CDs as a stopgap. If all else fails, some tunes on your smartphone through an 1/8'-RCA cable can at least put an end to the awkward silence during the most heinous of troubleshooting sessions. Most smartphones also have some kind of flashlight app built in, but it never hurts to have a small dedicated flashlight in your bag to navigate twisted cabling in dark and often tight quarters should the need arise. In short? Bring backup!
At big festivals, some kind of stage management will normally be on hand to make sure everyone stays on schedule, but even at bigger clubs DJs are often left to their own devices in terms of switching over. Relinquishing the reins to a big club system usually isn't exactly a DJ's favorite part of the job, though it is inevitable'"unless, of course, it's a closing set.
As your set time nears, a good practice is to approach the current DJ during a break in the mixing action within ten minutes of when you're due to begin, and simply ask how many more tracks they want to play. This is a polite way to make them aware you're due on soon, while empowering them to map a suitable exit from their set without rushing too much. Usually they'll ask what time it is, and rarely insist on playing more than two tracks at this point'"unless they were forced to start their set late for some reason.
If you're using a digital vinyl system or other computer-based DJ rig that requires extensive configuration, you should try to get as much of your gear as possible'"soundcard, controller, etc.'"plugged in during sound check. That said, many clubs don't offer a sound check, so you'll have to decide the best approach here; sadly, it's not uncommon to witness a laptop DJ mess up the end of the preceding DJ's set by accidentally unplugging the wrong thing'"a nightmare for all involved.
So rather than potentially ruin someone else's set'"and start your own with a beet-red face'"you might consider starting with a long track or two on CD or USB while you get your gear configured during your own set time. Then again, for people that have done the soundcard set-up routine enough times it can be almost second nature, so the defining factor might be the size of the DJ booth and how much room you have to maneuver. Playing a high-stakes game of Twister with a complete stranger in front of a raucous crowd isn't exactly the most comfortable or dignified way to start (or finish) a set.
When wrapping up your performance, it's a good courtesy to put on a longer track to give the DJ after you some additional time to get their bearing and ensure everything is working correctly for them. There are few things more stressful than finally plugging your headphones into the mixer in front of a big crowd only to find there's barely 30 seconds left of playtime before you need to be mixed in. Instead, leaving at least five minutes of running time on your final track is a good benchmark'"and if there are any unorthodox tempo, time signature or other changes in your last track, spare the next DJ the indignity of ignorance and be sure to let them know; by the same token, if there's an important breakdown or other feature of your final track that you want the dance floor to hear unobstructed, let the next DJ know and they ought to respect it.
Whether or not it's a mixer you're familiar with'"but especially if it's a mixer you've never used before'"don't be afraid to ask the previous DJ a few questions to make sure you understand the layout and routing; even if it's a model you've used a dozen times before, you want to make sure you understand how they've got everything plugged in.
Now that you've got control of the rig, it's time to calibrate everything for optimal performance.
Using CDJs? Make sure the tempo range and jog adjustments are where you like them. Playing on turntables? Get that tone-arm set up right.
Perhaps most importantly, you'll want to gauge the monitor speaker volume and set it to a healthy level, then adjust your headphone volume accordingly to ensure a good balance between the two; overcompensations in either direction can lead to accelerated hearing loss in the long term and a more difficult time beat-matching in the short term, so this is crucial. If you have a proper dual monitor setup, you might want to switch headphone ears each track you play to maintain a proper balance: right monitor, left earphone; left monitor, right earphone'"this way any potential hearing damage will at least be evenly distributed.
You're probably good to go at this point, but with the chaotic nature of a particularly fired-up venue, you never know what might happen. Just make sure you have all your backup materials easily in reach, and a friend or contact at the club ready to help out in case something out of your control goes wrong.
Ready for the best but prepared for the worst, it's finally time to let loose and get into the groove'"just don't forget to use the wash closet before you start.