I have to agree with tyroman, something that I have picked up from my teachers and has helped my white/chalk board writing, its give yourself points of reference.
For example, one teacher I have teaches in a classroom with two chalkboards laid side by side along the same wall.
Every time lecture starts, he begins by dividing the leftmost board in half with a single chalk line, and then writing to the right of that line, next he uses the unused panel on the leftmost chalkboard. When he gets done with that panel, he splits the right chalkboard and continues.
Through out the lecture he can erase panels as he needs them depending on whether or not they will topics in the remaining lecture(things like theorem statements, their proofs, etc.)
I have noticed that when I do the same things, my writing tends to come out in straight lines(parallel to the top/bottom of the board) and the letters tend to remain the same size and not begin at a readable size and taper off as the sentence goes on.
I would also recommend trying to obtain a white/chalk board that you can do some math on, you could then get more familiar with writing math, writing math in that size of font and writing on a white/chalk board in general