Such is human nature, especially when your subconscious is telling you it is something you don't want to do. Aside from using the tips here to make it so interesting that it is at the front of their mind all the time, ways to avoid this include having a totally fixed routine and schedule for homework, giving them a written schedule for all the homework at the beginning of each month or term, having the homework written up somewhere they can easily check it like the school notice board or blog, and checking that each person has marked the right exercise with the date it must be finished by