I take the summer and plan the entire year. I plan by week so that I have all 36 weeks planned out before school starts. On Sundays, I give the kids their schedule for the week. We have a daily schedule that is always the same, except for Fridays. Fridays is art/music, projects, and catch up. If there is anything that hasn't been done for that week it is caught up on Friday.
My kids are involved in time intensive activities (for example, my 10yo dd is at gymnastics for 17-20 hours a week). They know if they fall behind activities will go. School comes first. I am diligent to make sure everything for each week is completed. This takes self-discipline on my part. I find if I make my expectations clear and non-negotiable they buckle down and get it done. It is my job to make sure they stay on track and they understand that school gets done no matter what.