Here are a few guidelines for rehearsals.  These are not magic formulas but principles to help guide your own thoughts and ideas.

• Sing, Don't Talk
  Singers like to sing...let them.  You will be at your best when you are succinct and clear with your comments.  They will not improve by listening but by doing.
   

• Plan, Be Flexible
  The best thing you can do is know how every minute of the rehearsal should be used before you begin.  When the rehearsal starts to go a different direction, MAKE ADJUSTMENTS to your plan but don't abandon it.
   

• Rehearse Singers, Not Music
  Remember that these are people.  They sing because it strikes a deep point of resonance inside them.  Let the music speak to them.  They need to make music every day.
   

• Pace the Rehearsal
  Begin with familiar sections or pieces before digging into problems.  Build their confidence and then challenge them for more.  Send them out with a musical moment that will make them want to come back for more.
   

• Program for Success
  Before they start singing they should know why.  Focus their attention on it, then they know what they are listening for.
   

• Rehearse in Layers
  When you stop, fix one thing (sometimes two) and then rehearse it.  Then fix another and rehearse it.  Layer their understanding and ability one piece at a time.