Novelty vs Familiarity

Do you play mostly familiar favorites, or mostly new music that hasn't been heard before?  Or a mix of both?

I strongly recommend a mix of both.  Every group of dancers is different, so this isn't an absolute rule, but most dancers prefer a mixture of familiar favorites and new music.

Everyone has their favorite tunes that make them happy.  And people love having a chance to dance to popular tunes they've heard.  So don't leave those out.  Furthermore, the better leads want to match their variations to the breaks in the music, which only happen when they know the break is coming.  Similarly the better follows like to add stylistic flourishes and footwork modifications to match the musical changes.  So the more experienced dancers, both lead and follow, have a strong preference for familiar tunes, for these reasons.

Then new music is always exciting for most people, including the experienced dancers, so also include those in your mix.

DJ's more often err on the side of not enough familiar favorites (or worse, none).  Those are usually the DJs who are grandstanding, showing off their immense music resources.  That's their version of "Look at me! Look what I have!"  instead of selflessly giving the dancers what they want.  Then they wonder why the dancers don't come back.  But erring the side of only old favorites isn't much better, in my opinion.  Give them plenty of both.

This recommendation is specifically for social dance music.  If you're a psy-trance or techno DJ, then your following will want the latest music they've never heard before.  Conversely some dances have period themes, like all-fifties, disco or all-eighties.  And a wedding couple may request only their old favorites at their reception dance.  Each situation is different.

Variety and Pacing

You want to give your dancers a contrast between high and low energy.  Obviously you don't want to play two tunes in a row for the same kind of dance, at the same tempo.  Have you ever danced to a big band that played three swing or foxtrot tunes in a row with the same tempo?  The bandleaders were clearly not dancers.

Be aware that dancers will want to catch their breath after a fast tune, so maybe follow it with a slower one.  And they don't want to be lulled to sleep with too many slow songs in a row.  But you don't have to strictly alternate fast and slow dances.  Some DJs like to build the energy over several dances, bringing the dancers higher and higher but without exhausting them (yet).  Similarly, a string of several quiet dances can effectively set a deeper mood while also building up a desire to be hit with a high-energy set.

The art is to find the perfect pace, without too many fast or slow dances in a row.  If you want a safe rule of thumb (which has exceptions), don't play more than one or two really fast tunes in a row, and don't play more than two or three slow tunes in a row.

Songs of right length

You usually don't have to worry about your tunes being too short, but in the rare instance of having a favorite tune that's only one minute long, you can make a double or triple-length version easily, splicing together repeats of that tune, into one longer version, while maintaining the beat.  Music editing software is a good way to do this.

The far greater problem is a tune lasting too long, usually meaning more than three minutes.  It's a twofold problem.  (1) Most social dancers would prefer to have four 3-minute dances with four partners than two 6-minute dances.  Variety and contrast are good.  (2) Many leads/men feel that their repertoire of freestyle dance figures is exhausted after two or three minutes, and they would much rather move on to something else.

Since many recordings are longer than three minutes, you have two choices: fade the song out at about three minutes, or edit out a central part to retain the original ending.  I always listen to the original ending to see if it fades out, as much popular music does.  If so, then I'll know that an earlier fade-out at 3 minutes is okay.  But if there's a definitive ending (which is especially important in swing and tango) then I'll edit out some central strains to bring the song down to three minutes.  If the song is four minutes long, I'll listen through the piece several times, searching for the minute which I can remove from the center without harm to the song, usually a verse and chorus among many repeats of that verse-chorus.  If the lyrics are telling a story, you might worry about removing an important middle part of the story.  Don't worry — dancers are too occupied with dancing to be listening to the lyrics.  Once again, there are plenty of software options for editing middle sections out of music to make a 3-minute dance version.

Exception 1:  Some fast and exhausting dances might want to be shorter than three minutes, like a fast techno polka for instance.

Exception 2:  Some dance traditions favor long songs, like salsa.  And some groups are so specialized, like West Coast Swing clubs, that the leads know hundreds of variations for that one dance form, and are happy with longer songs.  Good DJs ask what the dancers prefer, including song lengths.  Dancers' wishes always come first.