The advent of technology which supports the concurrent presentation of information through a range of different media has raised new issues relating to the design of usable systems. While previous work in the areas of both Human-Computer Interaction (HCI) and hypermedia system usability can contribute a considerable amount to the development of such guidelines, we believe that the use of multiple output media demands an understanding of particular characteristics and limitations of users' attentional capabilities. This paper presents some initial guidelines for the design of usable multimedia systems. These guidelines are based on empirical findings regarding the nature of human attention derived from the field of experimental psychology. We believe that the provision of such guidelines for multimedia interface design will support designers in achieving the dual goals of maximising a user's flexibility in controlling the presentation of multiple concurrent media, while keeping cognitive load to an acceptable level.