When you're playing a tune for a dance, make the parts contrast sharply. I learned this by not doing it.
I was trying to work out some new technique, and had used the entire set for the last dance before the break to practice it. The fiddler knew the tune well, so I wasn't worried about any of my mistakes' throwing him off. As I was getting coffee, afterwards, a dancer, Bill, walked over to me and said, "Those last tunes were really confusing. I couldn't tell where the parts stopped and started."
I realized my mistake, thanked him, and apologized. It wasn't the tunes, it was my playing.
You can't expect dancers to recognize fiddle tunes. This is an advantage, since you can play the same sets, dance after dance, and they won't be bored. What they do notice is the changes -- transitions between parts and transitions between tunes. Key changes will elicit whoops from the dance floor.
Even when you're not changing keys you can still create easily recognizable contrasts. Back-to-back tunes with different feels, and tunes with sharp part contrasts is a good start, but you can also make each time through sound really different.
This is hardest to do when you're playing lead. Fiddlers shouldn't have to worry about anything but getting the melody out there. It's the other instruments that make the change. Guitar players can shift from open to closed chords. Other instruments can drop out and come back in. Once a tune or so, the fiddler can let the harmonica or hammered dulcimer take a lead.
I'll change the texture of a tune, on mando, by playing chords the first time through a tune, low harmony the second, melody the third, a drone the fourth, and so on. I try to make these changes crisp, and at boundaries.
Volume changes offer another good option. When the caller holds up three fingers to say, "Three more times through," use the first of those to let everyone know that the next time through will be soft. That's mezzo piano for you music readers, perhaps with a gradual build (crescendo) in the B2. The last time through, come screaming back in (forte) for a big finish, followed by applause from the floor.
All this time, the dancers are, at best, paying attention to the fiddle's melody. They'll probably not know what's making the parts sound different, but they'll still hear the changes. The dance figures change at these boundaries, too, and the texture changes give them the cues they can hear, to know when to go to the next move.
You won't sound any better as an individual musician, but you'll sound a lot better as a band.
