The growing interest in fitness services is demanding optimal service management and operation. This research aims to identify the best practices in fitness center management for increasing customer engagement and for long-term stability and sustainability of the business. A qualitative approach through 23 semi-structured interviews to fitness center managers was performed in Spain, covering private centers with diverse business models (7 low-cost, 13 mid-market, and 3 premium). The managers were asked about their characteristics, the fitness center characteristics, and the managerial decisions and individual opinion. The results showed the best practices divided into five sections: customer service, offered service, marketing, facilities, and general terms and conditions. Kind customer service, the existence of loyalty programs, enough tangible quality, and online marketing strategies are essential for fitness centers’ success. Additionally, a fair fee and a maximum of a 15 min trip to the center should be guaranteed. This information has clear implications for promoting sporting habits and for engaging more people in physical activity, complementing the extensive quantitative data in the existing literature.