My oldest preschooler is only three years old, so I'm not planning on focusing too much on handfwriting. However, I do want to begin introducing to her the concept of how letters are formed. Amazingly all the letters of the alphabet can be made with a combination of curves and sticks. To make the process more interesting, I've made a series of curves and sticks in fun patterns. I'm planning on printing and laminating them, but you could make them magnetic or with felt backing depending on your preference.
Here are the sets I've working on (click to download):
Radical Purple (pictured)