To efficiently track the reasons why opportunities are being lost, the consultant should create a new custom field on the Opportunity object. This custom field can be used to capture standardized reasons for lost opportunities, allowing for consistent data entry and easy reporting. This approach is straightforward to implement and leverages the existing data structure, ensuring that the information is readily available for analysis and strategic decision-making. Salesforce best practices recommend using custom fields for tracking specific business metrics to enhance data visibility and accuracy.