I decided I hated having chicken-scratch handwriting around the end of high school / beginning of college. I literally did writing worksheets (tracing over letters) like I was in kindergarten. And if I wrote a messy word / letter on my homework I'd cross the word out and do it again. It made a huge difference, and I started getting compliments all the time on my writing. It's slipped back into average territory since, but it definitely works!