Use a kitchen timer!!
Make sure your choose a time that is good for both of you so either before or after dinner or maybe just before bed.
Start with a couple of minutes, even if you child is not done his/her work/reading tell your child the time is up.
Then increase one or two minutes until the child builds 10 to 15 minutes of sitting down and doing work or even just reading for 15 minutes.
Make sure you don't push your child to sit for more than 15 minutes at a time. They can always finish their work the next day.
Make sure they pat their own back and get a bear hug from you when they have completed their work!