Set an alarm and push yourself to wake up early. Whether you get an hour of work done before the house is busy with children running around and talking constantly or you just use the morning to shower in peace and enjoy your coffee, you will be glad you did. Starting your day off well motivates you to get things done throughout the rest of the day.
Taking advantage of nap time for younger kids or right after bedtime is also a sure way to get some work done. It does mean you need to be strict about bedtime though!