Provide specific goals and performances to work toward. If the actors don't know when they will be performing and why it is important, they will be less motivated to act.
Plan trips that allow the actors to perform outside of the home town. Planning a tour to visit a nearby state can help encourage the actors to prepare with earnest.
Enter the group in contests. If the actors know they are going to be judged and competing with other groups, it will increase the odds of them attempting to perform at a high level.
Provide incentives for an actor to learn her lines. If she learns her lines and performs at a high level, offer a raise or additional roles that display her abilities. Offering additional solo parts and letting the actors audition for them can be motivating.
Arrange for the actors to have master classes and seminars with well-known and prominent actors. Getting the chance to work with professionals from local universities or theater groups will help motivate and encourage an actor to develop his craft.
Allow the actors the chance to vote on what musicals will be selected for the upcoming performance year. If the actors feel that they have a say in the direction of the group, each actor will be more inclined to work toward common goals.
Encourage the performers to attend summer camps in which they can learn additional acting techniques. If your group can afford it, offer to provide a few actors with scholarships or a reduced fee to attend the camp.