The Width tool (Shift-W) lets you drag strokes wider or narrower on any part of a path. You can add multiple width points to go wide to narrow to wide again. The Width tool is easy-to-use, but don't let that stop you from exploring all the functionality possible with this tool via shortcuts. For example, pressing the Option (PC: Alt) key while dragging will allow you to create an asymmetrical stroke width.
Tip provided by David Macy | Layers magazine Tip of the Day