To maintain good employees, create direct and precise systems to have in place to do everything. Installs, repairs, office work, managing, driving, when to fuel vehicles, when to clean the shop, what to do during down time, when to show up, when they will be expected to work overtime, how to keep track of inventory, when to update inventory, when to restock inventory, when they can make or take personal calls. Basically, create an employee manual with your expectations, and have them sign it. Then give them a list of what you will provide - wages, bonuses, days off, respect, when and how they can get promoted or raises, etc. Give certain employees more responsibilities. maybe Billy is in charge of keeping track of service to vehicles. Johnny is in charge of keeping track of overall inventory between crews. David is in charge of keeping the shop clean and organized.
I've asked my employees to come up with three goals for this year. It could be learning a new aspect of the industry - pumps, two wire, locates, audits. It could be selling upgrades to make systems more efficient. It could be taking on some of the office responsibilities. Something that they can learn and achieve, instead of just being a monkey with a shovel.