I like to create challenges that push me to improve the way I control the pen. It's like exercise or stretching and it helps immensely.
If you draw doodles, sketch abstract shapes and fonts, you effectively practice drawing smooth lines, improve your hand-eye coordination and ability to space out letters and lines in your work. Here's one of my practice sheets. First I sectioned off 9x12" Strathmore drawing paper (you can do this freehand or with a straight edge). Then I watercolored, doodled and picked a topic for each section. In the fruits & veggies section, I brainstormed the names of everything I could imagine, writing the words in various fonts as I thought of them. Without any reference, it's hard to remember the lush variety of fonts that exist.
It's easy to create a collection of material for your escapades in line practice. Select fonts from a font site such as dafont and click on the font to see the full description and all of the characters. Print 10-50 fonts and staple together. This forms your reference material for line work. Select fonts that are feasible to draw. My favorites are fonts with clean lines like Champagne & Limousines by Lauren Thompson; check out her posts on typographic inspiration.
Another type of line practice involves getting used to new pens and markers. At a coffee shop last year, I practiced writing and doodling with a new gold ultra-fine sharpie marker on a page in an altered catalog prepped with black gesso.
Using your reference material, collect pens & markers and make a piece of art using your font practice or pen experiments as the focus. This is an exercise in creativity and line control, and like drawing mandalas, it can be sooooo calming.