By the time you reach adulthood, writing has become so second nature that many people don't even stop to think that they may be doing it wrong. For many, improper handwriting technique results in messy, unreadable writing. It can be hard for adults to learn the correct way to write after doing it wrong for so long. Yet with practice, it's not only possible to improve your handwriting, but also relearn the proper technique so writing correctly will feel like it comes naturally to you.
Find out if you're writing the proper way. A good way to tell if this is the case is to pay attention to whether or not your arm moves when you write. If your hand is doing all the moving and not your arm, that's an indication that you're not writing properly. Practice writing when moving your shoulder, elbow and arm. Your fingers shouldn't be drawing the letters but simply guiding them while the rest of your arm does all the hard work.
Watch your hand to see if you're holding the pen the correct way. Your pen should be held with your thumb and index finger. If you're using your thumb and more than one finger, or your thumb and your middle finger, your handwriting will be messy. Practice holding the pen the correct way when writing. It will take awhile to break your habits, but this will vastly improve your handwriting over time.
Hold your pen lightly. Imagine that you're writing with a feather quill that would snap if held too tightly. One of the main causes of messy handwriting is gripping the pen too tightly. This creates cramped lettering. By adjusting how hard you grip your pen, your writing will appear more relaxed.