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.
I would say the biggest thing that makes my boardwriting messy is rushing. I know you said you don't want to write more slowly, so my suggestion is to try this: write (and speak) more judiciously. Then you can write slower and more neatly, when you don't need to hurry. It takes practice, and requires more preparation, but with a little more thought and planning, you can often be more clean with your presentation, which allows your writing and speaking to become more clean. [Disclaimer: I don't always practice this, particularly when I'm in a hurry before class.]
Also, jakebeal's answers are good. The angle at which you write (both side-to-side and up-down) is also important, and when you're not in a hurry you naturally adjust your body to write at a more comfortable angle.
In addition to Jake's great answer, what helped me was a chalk holder (you can find them online for $5-10). One reason people write terribly badly with chalk is that either the pieces feel like the are about to break, or they are tiny. Using a chalk holder makes writing with chalk more like writing with a pen.
I'd also recommend having two or three loaded chalk-holders waiting in reserve; having to pause mid-lecture to empty out small fragments, reload a new piece, and regain the thread of your discourse is terribly awkward.