While this has helped with the formation of letters, the most helpful tool has been the grips from Pencil Grip Inc.
I was not instructed on how to properly teach handwriting and pencil holding while I was learning to be a teacher. Pencil Grip Inc. sends a helpful diagram with their pencil grips. This was very helpful for me! I was able to see that one of my students was not holding his pencil correctly. They have the perfect grips in their 3-step training set to help correct this grip. With the crossover grip this student can't tuck his thumb in and it has really improved his writing.
My other student enjoys using the pencil grip. She says it helps her as she works hard to write correctly. I really have never thought about how specific grips can impact and improve a student's grip.
If you are looking for a tool to help improve student handwriting, you should try one of the pencil grips offered by Pencil Grip Inc. They are reasonably priced so you can try some in your room!