Sticking to the back-to-school theme, homework is likely to become an issue for most families very soon. For many families, the quantity, organization, completion, and thoroughness of homework is a serious source of stress.
I humbly suggest that before the nagging or argument starts, that you sit down with your preteen or teen during the first week of school and have a serious conversation about homework - what the issues have been in the past, expectations for this year, and ways to get around any issues you have had in the past. As always let your child know that this conversation is happening because you really love him or her, want him or her to be successful in school, and are willing to help with that goal. If you agree with the premise, remind them that you are more worried about the learning than the scoring.
Lay out the school expectation that your child can expect 30-120 minutes of homework a day (depending on their age) and then talk about what the issues have been in the past. If it helps, talk to teachers that you and your child like at school to get some suggestions. Then ask your child what they think is going on - is it not having a set place or time of day or evening to complete the work, is the schedule too busy, do they feel like they study enough but still do not do well on tests, does the T.V. or computer get in the way? Do they have difficulty concentrating?
You may find that the child's study skills are not what you thought or that they need some reminders to use flash cards, memorizing help, or that there are really auditory or visual learners and need to study in a different way. If they have not been doing well in a particular class, try different ways of studying. Maybe they learn by listening, so try asking them to explain the concepts to you, or maybe they need help memorizing, so prepare flashcards and drill them each day during a time they are usually relaxed and happy. Maybe the issue is concentration and some meditation training can help.
Whatever method you come up with, try and arrange your life so you can be with your child(ren) when they study, be interested in the subjects and offer to help and even check their work when they are done. If just being close is enough, you can do your own work at the same table, gladly helping if they need the support.
When it gets tough, just remember that most children want to succeed and please the adults they love - they are not trying to do poorly and chances are good that if you set your mind to it, you can find a way to help them succeed. Good luck!