Maria Thomas does this lovely little thing in her zentangles, to add highlights.  Rick refers to them as Sparks.  It's simply a matter of making some lines shorter than others, but it's very effective.  I'm using Sparks here to spice up an otherwise bland pattern.  The reason I offer this otherwise bland pattern, is that it makes a great background.  It is also great practice for linework. 

The outer lines can be hard to draw because they are so long.  

Tips:Work on a steady surface. 
Look at where you want the line to end, not where you are currently drawing.
Keep your hand light.  If you press down, trying to control your pen, you just make yourself more wobbly!
Once you start having trouble, stop adding lines.  You can get bolder as you get better. 

Edited to add: On my flickr site post, Marguerite1997 left a tip that I hadn't considered.  She said "And remember to pull your pen, not push it. Little drafting tip."  The minute I read this, I knew she was right.