What are good tips/ways to improve my writing on chalkboards?

This is going to sound stupid, but I've seen it work. Try dividing the blackboard with vertical lines to create narrower "pages." This will make it easier to write in a straight line and will discourage you from writing too large. Of course, the "pages" need to be large enough that you don't write too small.

As you mention this is about the physical act of writing, then yes, there are things you can do.

Put a projector with text on various sizes (like the vision testing charts optometrists use) and walk to the end of the room to see which size is more fitting given the room size, then go back to the board and mark the size. You then can measure it with a normal ruler (there are BIG wooden rules and instruments for chalkboards too) and then proceed to mark at the edges of the board the lines for that letter size. Then you can get some threat and tape from mark to mark to make a whole board ruler to mark dots on the board (or place stickers).

Then it's practice, practice and practice.

Sectioning the data you are going to put on the board before hand in a notebook can help you plan the spacing and distribution. It's a good practice for the lecture but it can be time consuming.

Alternatively,if you want to help the students more then you can provide them with digital notes on the topic.

Maybe it's silly, but there is a section on Blackboard Technique in Gian-Carlo Rota's Ten Lessons I wish I had been Taught that I have always found very useful:

Blackboard Technique

Two points.

a. Make sure the blackboard is spotless. It is particularly important to erase those distracting whirls that are left when we run the eraser over the blackboard in a non uniform fashion.

By starting with a spotless blackboard, you will subtly convey the impression that the lecture they are about to hear is equally spotless.

b. Start writing on the top left hand corner. What we write on the blackboard should correspond to what we want an attentive listener to take down in [their] notebook. It is preferable to write slowly and in a large handwriting, with no abbreviations. Those members of the audience who are taking notes are doing us a favor, and it is up to us to help them with their copying.

When slides are used instead of the blackboard, the speaker should spend some time explaining each slide, preferably by adding sentences that are inessential, repetitive or superfluous, so as to allow any member of the audience time to copy our slide. We all fall prey to the illusion that a listener will find the time to read the copy of the slides we hand them after the lecture. This is wishful thinking.