Have a Backup Plan and a Backup Backup Plan
Along with your main digital player and its power cable, it's a good idea to bring a second digital player or a laptop - all loaded with your music.
Don't Rely on an Internet Connection
Streaming music services like Spotify are great for personal listening, but they can cause big problems at a wedding. If the internet connection goes out or resets, you'll be left without music. Instead, use offline music sources.
Prevent "Guest DJs"
There's always that one guest who doesn't like what's playing and tries to change things up. Instead of bombarding a regular DJ with requests, she'll be trying to hit shuffle on your playlist, or even switch the cable to her own device. But you can prepare for this bad wedding guest. First, ask for music requests, either on your wedding website on your reply cards with a line like "Name some artists or songs you're hoping to hear at our reception" or "What songs are sure to get you dancing?" Hearing a favorite song might satisfy a would-be guest DJ, but in case it doesn't, you should have a sound manager or "freejay" who can guard and hit play for any special songs. When the freejay inevitably needs to take a break, hide your device under a taped-down piece of paper that says, "We've chosen our wedding playlist carefully. Please don't touch the iPod!"
Crossfade and Cut
One of the most important things that a DJ does is create transitions between songs. Six seconds of silence might not sound like much, but it's certainly enough to kill a mood and empty a dance floor. You might also have a favorite tune with a long intro or an extended ending that just won't work. Fortunately, there are DIY DJ solutions to these problems. Some models of iPods allow gapless playback. You can also use iTunes to both crossfade and cut songs, and DJ apps like Virtual DJ or My Wedding DJ can help you with advanced techniques.
Think through each part of the wedding
Often when people decide to iPod their wedding, they are generally thinking about one part of the day — the ceremony, cocktail hour, or reception. If you're only using an iPod for one part of the wedding day, it generally has an effect on the other pieces. For instance, if you're using an iPod for the wedding reception, who will bring mics for the ceremony and reception announcements? And who will emcee the reception and make those announcements? There are always solutions to be found, but to avoid last-minute stress, it's good to think through the music, sound, and audio for the entire day, and plan accordingly.
Appoint a sound person
Pick one person (and it cannot be you or your partner, because you'll be busy, you know, getting married) to be responsible for setting up the equipment, and controlling the playlist and sound. Your sound person should know a thing or two about the equipment, sound levels, feedback, and all that good stuff to ensure that if there's a complication, he or she can troubleshoot the issue smoothly. The best appointed sound people I've seen at weddings are the ones who always take their role super seriously, and know how to prevent issues before they become a problem. If my husband and I were to get married all over again knowing what I know now, this time around I'd take my own advice and steer myself completely clear of the iPod during the reception, and leave it in someone else's capable hands.
Make a playlist for each section of the wedding
For the ceremony, you'll likely have two to three songs picked out, and your ceremony playlist can be as simple as that. The rest of the day can be chunked into playlists that fit the mood for that part of the wedding. Playlists can be made for cocktail hour, introductions, dances (such as the first dance, father-daughter dance, and mother-son dance), dinner, group dancing, and the after party.
Make longer playlists than you think you need
Make longer playlists than you think you need for the cocktail hour and reception, as those are the two parts of the wedding that can be challenging to predict exactly how much music you'll need. Sometimes things run long, sometimes short, but either way, you want to make sure you have some extra music at the end of each play list. For example, if your cocktail hour is one hour long, it's a great idea to make a playlist that's an hour and a half.
Build momentum with the music
As you make your playlists (particularly for the reception), think about how all the songs build on each other so that you eventually get to the point where no one can help but be on the dance floor. For instance, toward the end of dinner, make sure you're playing some music that puts your wedding guests in the dancing mood, then after that consider some music that's appealing to all ages when the dancing starts to get everyone on the dance floor. Then increase the momentum throughout the rest of the night.