Being careful who you give permissions is a start. Depends on the business you have. Don't give 100% access not even to the highest standing admin and avoid giving sensitive information about clients to anyone that doesn't absolutely necessarily need it. Do backups of all data on external hard drives and have them with you!
I know of a company that got tricked by its chief developer. The guy had a 20% share in the company and still deleted core files on the servers, stole the database, including the clients and opened his own company one month later.