Sit up straight. Posture is very important when it comes to your handwriting. You need to have your feet flat on the floor and your elbows resting comfortably on the surface you are using to write with.
Hold the pen correctly. Having the wrong grip on your pen will cause your writing to look sloppy. Hold the pen between the thumb and forefinger at a slant, with the tip of your ring finger supporting the bottom of the pen tip, and keep your fingers relaxed and not grasping tightly.
Write with your arm and shoulder, not your fingers. One of the most common errors people make when writing is to draw the letters on the page and to only use their fingers. Test yourself. If your hand is moving more than your arm and shoulder, you are a finger writer. Fluid writing comes from using your arm and shoulder muscles. Practice writing by keeping your hand on the page as much as possible.
Choose a slant. Another problematic aspect of poor writing are letters that go in different directions. Choose either a forward, backward or vertical position for your letters and stick to it throughout your writing piece, to give uniformity.
Practice problem letters. Look at your writing and pick out any letters that are consistently sloppy or uneven. Practice writing those letters with proper posture and holding of the pen. Write them slowly and use a large script at first. Once you become more accustomed you can write them smaller.
Practice. You should practice your handwriting every day if you want to see improvement in the quickest time possible.