Students will produce work to the standard you accept. Only accept a high standard of work.
Early in your first day with your class, set the standard of work you will accept by asking, "Is that the best work you can produce?"
If the answer is "No", then respond with, "Well, if it's not good enough for you, then it's not good enough for me. How about you have another go and show your classmates AND ME your best work."
If the answer is "Yes", respond with, "Let me help you improve. Use a sharper pencil, write more clearly, use a ruler for your lines .. " as appropriate