Messy handwriting is very common in children (and even adults!) long after they learn the basics of writing. Little or no spaces between words, or a combination of cursive and printed letters, are two common issues that some kids struggle with. Usually, a child's brain is working faster than the hand can move, so he or she tries to write down everything as fast as they can — resulting in illegible handwriting.
The first step to help messy handwriting is to take your child for a vision test to rule out bad eyesight as the culprit. If his or her vision is fine, timed writing exercises can help improve his or her handwriting at faster speeds.
Have him or her select a short passage — about three or four sentences — from their favorite book, and ask him or her to write it as fast as he or she can while you time them. Repeat this exercise one or two times a day for three or four days. Once he or she masters the passage, choose a new one and repeat the exercise. You should begin to see improvements. (A little reward after you've finished the exercise can also be a strong motivator to help improve, too!)