I found that the thing that affected my handwriting the worst was when I was studying Japanese. Now my handwriting is close to illegible in both Roman and Japanese scripts.

Writing with a ballpoint, my handwriting gets worse. I've found that I need to use a fountain pen to keep it legible (as long as it stays dry) because those pens require more attention to angle/pressure/etc in order to write. The pen I use is a Namiki retractible, which entertains some folks.