When I write on tablets and boards, I use a much simpler handwriting style, usually neat capitals, so I have writing for me and writing for others.
The main thing that kills penmanship is a lack of practice, like playing guitar. I think Nile Rodgers said, if I don't practice for a day, I know it. If I don't practice for two days, my band knows it, and if I don't practice for three days, the audience knows it. Maybe it was Pat Metheny, but the point holds for me. If I don't write in my diary for a few days, the writing isn't as smooth and fluid. But when I have been writing like a beast, the words flow like anything onto the paper.
Hard to find time to pick up a pen for pleasure.