In this paper we present a highly modular planning system architecture. It is based on a proper formal account of hybrid planning, which allows for the formal definition of (flexible) planning strategies. Groups of modules for flaw detection and plan refinement provide the basic functionalities of a planning system. The concept of explicit strategy modules serves to formulate and implement strategies that orchestrate the basic modules. This way a variety of fixed plan generation procedures as well as novel flexible planning strategies can easily be implemented and evaluated. We present a number of such strategies and show some first comparative performance results.