Good tip, Uri. Recent versions of Mylyn now display a message in the package explorer "empty task context, unfocus or alt+click" when you activate a new task.
As Uri points out, one way to get started is to unfocus using the toolbar button and work on your task normally for a while (with uninteresting resources automatically greyed-out). You can then focus the package explorer when you only want to see relevant resources.
Other users prefer to keep the package explorer in focus mode and hold down the "Alt" key while clicking in the package explorer to add new resources to the active task context. In this way of working, only the interesting resources will be visible, but you can always un-focus to see everything if needed.
