One of the best things a parent can do for his child is to teach him how to do his homework. A child who has reached junior high school should be almost completely self-sufficient in this area.
Kindergarteners are frequently given family-type projects to complete as "homework", and this is where a parent's instruction begins. Often these projects are due with a few days or a week, and they are provided merely to acquaint the child with the concept of homework itself.
A space within the house should be designated as the workspace, whether this is at a desk in the child's room or at the kitchen table. A time should also be determined, whether it's an actual time of day, such as 4:00 p.m., or a more ambiguous "after dinner".
The space is important - this should include all materials the child needs to complete his homework, gathered beforehand or kept there permanently. The time is less crucial; what matters most is teaching the habit of when to do homework.
As children become older, and homework more difficult and more frequent, the habits begun in kindergarten can and should be continued, with one notable exception: parental involvement should exponentially decrease.
Throughout the primary grades, a wise parent will sit down with the child and, in the early years, walk them through the papers and instructions. This will benefit the child in two ways: it will teach him the proper procedures, as in reading directions before starting, and it will teach him organization. The parent is present to coach, help clarify, and keep the child focused.
During the elementary years, a child may yet require prompting to ascertain if he does have homework, as well as to remind him to begin. He will probably also need to be occasionally checked upon and be able to ask questions of the parent.
At this stage, however, a parent does not need and should not actually sit down with the child, unless of course there is a difficult assignment which really does require assistance. The goal is to teach the child to do the work, the work is his responsibility; the parent has more than likely already completed this grade and did his own homework at that time.
If the parent has done his job, by junior high, and certainly beyond, the child should be able to do his homework completely on his own. Naturally, a parent will be available for questions, but a child's homework must be his own. There is a reason that teachers assign homework, and it's for the benefit of the child - not the parent.
If the child has a place to study, a time in which to do so, all materials available and accessible, he should be able to complete his homework in a timely fashion - all on his own. A parent should never have to sit down with a child over the age of twelve and coax him to perform.