What if there is a band onstage behind the actors, and you can't use wireless lavalier mics on the actors?
* Ask that the band play more quietly.
* Ask that the actors talk or sing more loudly.
* Move the band offstage or into a pit.
* Place the PA speakers closer to the audience.
* Use mics with tight polar patterns (supercardioid, hypercardioid).
* Notch out feedback frequencies with a narrow-band equalizer.
* Keep the number of open mics to a minimum. Turn up only the mic nearest the person speaking.
* Aim hanging mics straight down to reduce pickup of the band. Adjust the actors' blocking so they are close to the mics.
* Use stand-mounted or handheld mics on the actors. Have the mics be a part of the production.