There are a bunch of goals here. You want to give newer musicians a place where they can learn how playing for dances works and get some practice at it. You want experienced musicians to have a good time, maybe get to fool around some on instruments they don't usually get to play, be a bit sillier than usual. You want musicians to get to meet each other and hang out, for fun but also helping form connections that could lead to new bands. You want the music to sound good and the dancers to have fun, so that open band nights are something to look forward to and the band is one people want to be in.

Your typical contra dance band is three or four people who've played together a lot before, probably with some amount of practicing and planning. The instrumentation generally has some thought put into it, with a good variety of capabilities and sounds. Small size and practice combined with good listening and learned anticipation get you a good tight sound, and the challenge is often getting your small group to sound "full". Open bands are nothing like this. You've got a horde on random instruments who've never played as a group before, with a wide range of abilities and musical aesthetics. A tight sound is going to be work, but you do get "full" for free.

What do you do before the dance? The band needs musicians. Make sure your friends know about the dance, and let them know you'd love to get to play with them again. Think over the area musicians and bands for who else to invite. Without invitations you will tend to get fewer experienced musicians, who often need a bit more of a pull to get them to come out. The best pull here is their having had a great time at past open bands, but a personal invitation goes a long way. Getting at least one melody player and rhythm player to commit to come can make you a lot less anxious, and a lot less exposed to "huh; I guess everyone brought fiddles today". You can hire these as "anchor" musicians or not, depending on budget, organizational preference, and supply/demand.

The band also needs sound. It's possible to just throw up a few area mics and send the on-stage sound out into the hall, but don't do this. Some people are going to show up with loud/quiet instruments, you might have different numbers of different kinds of instruments, and some people are better musicians than others. Giving people individual mics gives you the control to build a balanced group sound. You don't have to mic everyone: ideally you have enough mics, stands, and channels that you can offer a mic to anyone who wants one, and lots of people will say no. The bandleader needs to work closely with the sound person here, both before the dance to make sure there will be appropriate equipment to mic everyone and during the dance to make sure the people you want to hear are properly miced. The sound in the hall should heavily favor the experienced musicians, but variety in timbre is important too: if your only accordion player is newer than five of your fiddle players you want enough accordion in that hall that people do hear the different sounds.

As people show up on stage, tell people where to sit and then ask people to move when circumstances change. Get the rhythm musicians to sit together on one side, around the piano if there is one, so they can hear each other well and call chords. Get your best melody musicians together at the front where they can also talk and egg each other on. People probably won't be good at showing up on time, so at first just have people sit together on part of the stage, and then as more people come in have people move over and make room. This might feel a little pushy, but having musicians scattered all over the stage makes for a sound as uncoordinated as the stage looks. It might also feel elitist doing a bit of sorting people by ability, but newer musicians getting to play in a thriving open band that has some really good musicians and plays really good music is more valuable than their getting to sit up front.

With rhythm instruments there's probably going to be some contention. Generally you'll only have one piano and, while you can have multiple guitars, guitarists are usually used to handling the rhythm alone. You're in a weird situation where a lot of your best rhythm musicians are used to being in full control of the chords and being free to play whatever they think would best fit the moment. This is really valuable, but also hard to work into an open band if the guitar player is thinking one groove and one set of chords while the piano player's head is somewhere completely different. Rotating musicians through on the piano can be helpful, but also it's ok to tell people "for this set, person X is in charge, listen to them and follow their lead". Alternate instruments are also helpful here: piano players can play piano accordion, a guitar player can play bass. When I can I like to bring extra instruments that people can fool around with and figure out. And people can go dance.

Your most powerful tool for coordinating rhythm musicians is the same as your tool for making the band sound like a group of people that have played together and made plans: actually leading. Tell people what to do. Ideally you have enough musicians that you don't have to play anything yourself except occasionally to add flavor or convey a musical idea you want people to follow. This leaves you free to think about what the band should do and then communicate that.

If you just let the band play, they'll pretty much go through the tune the same way every time. Dull. So you stand at the front, turn ideas into plans, and make sure everyone knows what's happening. Some ideas we played with Sunday, in increasing order of weirdness:

• Just melody
• Just fiddles
• No fiddles
• Specific person plays melody solo (make sure they're mic'd!)
• Quiet
• Drone on one chord
• Building/Crescendo
• Heavy downbeat emphasis
• Tell people how to end: long, short, choke, scottish
• Call chords to the rhythm musicians (tell them what you're going to do, then call the chords as you go)
• Call chords to the whole band, no melody
• Start with one instrument, bring people in one by one.
• Build a chord piece by piece (E7 over the B part to Evil Diane)
• Pick a quote (Wimoweh over Calliope House) and get some of the melody people to play that while others stay on the melody
• Singing instead of playing

When you have an idea you want the band to do, stand in front, wave your arms and be loud. Don't be afraid to be goofy or get really into the music: the more you do that the better they'll play. It can be useful to make signs and hold them up, some people see better than they hear, especially while playing.

(One thing I really like about open bands is you can be sillier, dorkier, and more dramatic than named bands. Named bands want to be cool, want to be respected, want to be remembered. Open bands just want to give people a good time in the moment. And tasteful restrained subtlety wasn't really on the table anyway.)

Update 2014-10-07: I realized I didn't say anything about tunes. It's good to distribute a tune list ahead of time so people can practice. Most open bands have a long tune list that they then play a small fraction of at the dance, but that's coming at it the wrong way around. If someone practices tunes from a list you want them to get to play them at the dance, which means you should have a short list and try to play everything on it. Then as the moment feels you can add a few other tunes that people are excited about. You want to pick well known tunes, even overplayed tunes, so the most people are comfortable with them. After all, they got to be overplayed via being awesome. Don't completely avoid notey complex ones; without any of those the dancers will feel the lower energy and your experienced melody musicians will miss them. It can work well to mostly do accessible tunes but begin and end the halves with denser ones.