I used to have pretty poor handwriting as well. It didn't help growing up in school when learning to write they would just have me trace letters, and never taught what actually makes handwriting legible. A few years back I started practicing calligraphy for fun and it helped out quite a bit. The basics that translate to everyday handwriting are:
1.Use your big muscles to write.(Move your shoulder/elbow, while keeping your wrist and fingers relatively stable.)
2.Make sure vertical lines are all on the same angle. Vertical parts of "l's","h's","T's" etc...
3.The round parts of letters are the same shape (For example the rounded parts of "o's","e's", and "c's" are all the same)
1.Use your big muscles to write.(Move your shoulder/elbow, while keeping your wrist and fingers relatively stable.)
2.Make sure vertical lines are all on the same angle. Vertical parts of "l's","h's","T's" etc...
3.The round parts of letters are the same shape (For example the rounded parts of "o's","e's", and "c's" are all the same)