Having good handwriting is very useful if not down right important for a good number of jobs out there. Especially if someone else needs to read your handwriting, then it is hopefully at least readable. It is never too late to improve on your handwriting though, no matter what you think. It is as simple as practice makes perfect, but what should you practice?
There are a few tips that you should practice with so that you are writing properly. Most people aren't taught quite what a proper writing form is and this is usually the underlying cause to someones poor handwriting.
Adjust Your Grip
Most people hold their pen with their thumb, index, and middle fingers. This is ok, you should go with the grip that is most comfortable to you. People are different and holding it one specific way might make your hand hurt while another person it could become quite comfortable.
Use the Shoulder Grip
Most people who have poor handwriting have an issue where they hold the absolute edge of a pen or pencil when they are writing. This is how you draw, not write. The difference being that when you hold the pen back on the shoulders you use more muscles that can sustain the writing motion for longer and more accurately if trained. You use the muscles in your shoulders and a little in your back, they are more able to write accurately and for longer than your wrist and forearm by far.
You can train your muscles to write like this better by simply holding your arm out and pretending to write in the air. Progressively write smaller and smaller in the air, you will have no visual indicator as to what you are actually doing but this is actually a good thing because how often do you write something based on visual ques? Exactly, you know how to form all these letters already, you are just training your arm into doing it better! You should do these exercises for 5-10 minutes every other day for a week or so, or until you achieve the desired results.
Remember the best way to get better at writing is by taking your time and trying to write as nicely as possible. Then occasionally trying to write really fast and comparing. Take note of what needs improved, get in a groove for writing. You should only do this for about 10 minutes every day, take it slow and steady and you will improve your handwriting.