When you create use case diagrams in the UML, remember that every use case diagram is just a graphical presentation of the static use case view of a system. This means that no single use case diagram need capture everything about a system's use case view. Collectively, all the use case diagrams of a system represent the system's complete static use case view; individually, each represents just one aspect.
A well-structured use case diagram
Is focused on communicating one aspect of a system's static use case view.
Contains only those use cases and actors that are essential to understanding that aspect.
Provides detail consistent with its level of abstraction; you should expose only those adornments (such as extension points) that are essential to understanding.
Is not so minimalist as to misinform the reader about semantics that are important.
When you draw a use case diagram,
Give it a name that communicates its purpose.
Lay out its elements to minimize lines that cross.
Organize its elements spatially so that behaviors and roles that are semantically close are laid out physically close.
Use notes and color as visual cues to draw attention to important features of your diagram.
Try not to show too many kinds of relationships. In general, if you have complicated include and extend relationships, take these elements to another diagram.