Practice gripping the pen as you write so you are holding it neither too tightly nor too loosely. Your handwriting becomes shaky the tighter you grip a pen, and the fingers should be used to guide a pen rather than being used to create the letters.
Take your time as you write. When you write in a hurry, you are less articulate with your penmanship.
Pay attention to your writing. Don't let your mind wonder and daydream, and don't pay attention only to what you're writing. Pay attention to the letters you are forming. The more aware you are of your writing, the better your handwriting will be.