Visual "breaks": Try to set up homework near a window if possible, to let your child lift their eyes and take a little break if needed. The visual space should not be too busy to reduce distractions. The area should also be well lit. Brighter lights should be avoided close to bedtime. Calming colors like as ocean blues or soothing greens are helpful.
Seating: This will vary from kid to kid; some kids are fidget prone so do well with a seat that spins, rocks or bounces. It might drive you crazy, but it may be just what they need to help keep them focused. For other kids, a supportive chair and desk may do the trick. Still others may find it helpful to lie on their bellies on the floor. The important thing is that they are in a position that lends itself to individual learning.
How near or far? You may need to keep smaller kids close, such as at a kitchen table to counter to monitor how they are doing. Older kids mostly do better further away from the distractions of family, but still need to be checked on occasionally.
Supplies: These should be close by to minimized getting up and down and thereby distracting your child. They should be kept in neat containers with easy to find items. Stressful homework is made more stressful when they can't find a number two pencil.
Brain breaks: Kids should get up and stretch or move around periodically if their homework time is long. This depends on the kid; some need to get up and do a few jumping jacks every few minutes. Others may be every half hour.
Shared spaces: If your kids are constantly at each other's throats lately, don't expect that to stop just because it's homework time. Give them their own spaces or have them work on their homework at different times.
Music or no music? The consensus of most studies on the topic is that music without words is better than loud music or music with words. However, keep your eye on your teenager. If they are getting their work done even with earbuds, it may not be worth the battle.
Good luck with homework!
Remember to keep the lines of communication open with your child's teach about homework.