Poor posture: Don't sit on your leg while writing, feet flat on floor, knees should be bent at 90 degrees and arm resting on desk (not on top of books or binders), bottom corner of paper slants to left for right handed writiers and to the right for lefties.
Poor pencil grip: Pencil should be held only with thumb and index, resting on middle finger, ring and pinky, curled into palm. Try using different grips to help especially if your hand hurts or you get calluses from too tight a grasp.
Too fast a pace resulting in letters not finished or letters floating or sinking: Use graph paper which will slow down writing speed
Reversal of letter formation - b,d: Lower case b starts at the top and d starts in the middle from a magic C , and not making letters from the bottom-top middle only.
Not staying on lines or writing too big - especially on an unlined worksheet: Highlight lines or create boxes for writing within specified boundary.
No spaces between words - Try a small paper clip to provide spaces between words.
Writing outside of margins or not starting at left margin: Vertically highlighing margin line.
Too dark, too light: Experiment with different types of pencils, pens or pencil grippers.