Techniques for good board handwriting
I have struggled with this too.  Two things that I find help are:


Write much bigger than you think you should.  It's easier to be neater with bigger letters.  You can fit less on a board, but honestly that's generally a good thing.
Move your body along with your writing as you go.  My writing gets worse the farther my hand is from my center of mass, and tends to trail downward too.