Conducting periodic user awareness training is the best way to mitigate accidental data loss events because it can educate the users on the causes, consequences, and prevention of data loss, and increase their awareness of the security policies and procedures of the organization. User awareness training can also help users to identify and report potential data loss incidents, and to adopt good practices such as backing up data, encrypting data, and using secure channels for data transmission and storage.
References: The article Mistakes Happen-Mitigating Unintentional Data Loss from the ISACA Journal 2018 states that "user awareness training is the most effective way to prevent unintentional data loss" and that "user awareness training should include information on the types and sources of data loss, the impact and cost of data loss, the legal and regulatory requirements for data protection, the organization's data security policies and procedures, the roles and responsibilities of users in data security, the best practices and tools for data security, and the reporting and escalation process for data loss incidents" (p. 2)1. The Data Spill Management Guide from the Cyber.gov.au website also states that "user awareness training is an important preventative measure to reduce the likelihood of data spills" and that "user awareness training should cover topics such as data classification, data handling, data storage, data transmission, data disposal, and data spill reporting" (p. 2)