How do I get my child to be more disciplined about homework?

Expectations. Set expectations, and a specific schedule. Your child should not be allowed to participate in desired activities until the homework is completed.
Set a schedule. Children thrive off patterns and quickly a set schedule will become a habit. After school or sports practice have a healthy snack (fruit and yogurt, nuts, etc) and then have homework time. Have a set place in your house to do homework (good lighting, limit distractions) only after this time should a tv show or video game be allowed. Doing homework late at night is not healthy or productive.