It's good to know by heart what the longest notes, especially the ending notes, of the song are. Then you pick up, down and around them, finally landing on the same note as everyone else to end it. Say the words of the song in your head as you do this.

Be sure you know what key you're in and stick to notes in that key.

If something sounds awful, play a new note ASAP.

It also helps to have the improvisation considerably higher than the melody. That way collisions are less noticeable. I'm told that bluegrass descants are a fifth above the melody.