A great place to start is by Organization. Being well organized is vital to a healthy and happy music ministry. An important thing to consider when you have dedicated volunteers is making sure everything they need is available for rehearsals and services. If you have a band, pulling their charts ahead of time is important. Same for choir or worship team. There's nothing like waiting on people trying to find their music when you're already late getting started.
Which reminds me - starting on time is important too! Starting on time says you are aware and appreciative of everyone else's schedule. Not to mention it gives you more time to prepare.
Speaking of preparing, planning a rehearsal ahead of time is helpful. How will you welcome everyone? What song will you start with? Will you include a devotional moment? Will you make announcements? If you wait until rehearsal time to figure everything out you'll end up with a rehearsal that isn't all it can be.
There are many examples of how to run a rehearsal. However, here are a few thoughts for a great rehearsal in no particular order. Welcome. Review a song from last rehearsal. Work on something new. Worship together. Pray together. Fun moment, like "Hello my name is...". Start on time. End on time. Prepare an encouraging word or a short scripture. Get creative with other ideas.
Remember, these people drove several or many miles to attend rehearsal so try to make it memorable. Also, pray for them throughout the week and encourage them to do the same for each other. Prayer still changes things for the better.