DC-DC converters are key topologies in many industries such as renewable energy, transportations, utility, and perform an increasingly important role in the power electronics. The research of the modulation strategies with efficiency improvement is therefore critical for the DC-DC converters. As one of the most classic DC-DC converters, the dual active bridge (DAB) converter has attracted more and more attention in the utilization of the isolated DC-DC power conversion systems, due to its good performance. Over the past decade, many advanced techniques have been used to promote the efficiency for the DAB converter. Aiming to provide comprehensive analysis of different optimizing techniques for the DC-DC converters, this paper gives an overview of the efficiency optimization modulation strategies with a case study on the DAB converter. Firstly, typical phase-shift modulation methods and variable frequency modulation methods for the DAB converter are analyzed. After that, the application of three categories of optimization techniques are described, which including numerical optimization methods, metaheuristic methods and reinforcement learning (RL) methods. On this basis, this paper provides corresponding suggestions for further improving the operation efficiency of the DAB converters.