The first way I can think of to get those lines is to use Illustrator, and stroke the lines with an art brush.
That might not be feasible, however, so if you have PS7 or greater, there are some controls in the brush palette that might help you out. In the brushes palette, you can set "Shape Dynamics-Size Jitter-Control" to "fade," so that the farther along in stroke, the smaller it will get.
Or, if you have a graphics tablet, such as one made by Wacom, you can set the size of the brush to be controlled by pressure, which will give you very precice control over this.
Lastly, you could create paths using the pen tool, set up a brush, and then when you stroke that path, there is an option to "simulate pressure."
If any of those sound like what would help you, post, and I can explain it in more detail.