As a standardized NGN architecture, the 3rd Generation Partnership Project IP Multimedia Subsystem (IMS) defines standard service capabilities and interfaces, as well as a common IP-based infrastructure, which enable operators to introduce new services in an efficient and flexible way. In order to exploit the capabilities offered by an IMS operator, multimedia application developers face a business challenge to create applications which can run on a variety of user devices, and possibly adapt to various network conditions and user preferences. Our goal was to identify common generic functionality related to session-level QoS signaling for advanced multimedia applications and to design a high-level application programming interface (API), which invokes this functionality. The paper describes the Dynamic Service Adaptation model on which the proposed API is based, the API specification, and its implementation in Java. A prototype multimedia application featuring a 3D networked game provides an example of using the API.