Improve handwritting

[Feathered fox ink]

I agree with Impractical Cartographer here, writing with a nib could help you. You won't write as fast as with your usual pens but it makes you more carefull about the different stroke to trace a letter.
Calligraphy books can be useful, too, with explanations on how to trace the letters. Even if you don't plan on using actual calligraphy on your maps, it would make you think more about the shape of the letters: once you understand it, you can try adjust it to your own style.

[etrnlflame]

I end up cheating and uploading the map to my computer where Illustrator can write for me...