Appeal to Their Motivation
If your child is not doing his chores, find out what their plans are after they've finished and try to motivate them toward getting the work done so they can move onto what they really want to do. Appealing to a child's self-interests - rather than explaining their duty or responsibility - is usually much more effective.
There should be a consistent time to do chores in the evening or morning. Evenings are best during the school year, as doing chores in the morning adds to the stress of their schedule. It's a good idea to set aside a time during the day when all the children in your family are doing their chores at once. Your 15 year old might be unloading the dishwasher, while your 11 year old is taking out the garbage. This way, no one feels as if they're being punished or missing out by having to complete their tasks. It's just chore time. Chores should be done before anything else gets done. Before any electronics go on; make it a rule that your child's bed has to be made, their clothes put away and their room tidied. They need to learn that, before they can have free time, their responsibilities have to be met. Again, you want to get them to do the boring and mundane things to get to something exciting.
Give a Chore Based Allowance
Your child's allowance should be connected to their chores and to the times when your child fails to complete their tasks or has to be reminded to do them. For example, if your child has to be told more than once to do their chore, they would lose a certain part of their allowance. It is appropriate to give that part of their allowance to a sibling who does the chore instead. This way - you're working on the communications process, as well as your child's motivation.
Implement a Reward System
Put a chart on the refrigerator with each child's name on it and their respective chores. If they make their bed correctly and promptly, they get a check. When they get five checks, they get some reward. Maybe it's staying up an hour later. Maybe it's having more computer time one night. If they want more than that hour, they should have to earn it. This allows you to use computer, TV and video game time as a reward.
Positively Time Your Child's Performance
This technique motivates children to compete with themselves. You say "Let's see if you can get it done in 15 minutes tonight. Remember, you have to do it right. I'm going to check. If you get it done within 15 minutes, you can stay up 15 minutes later or you can stay online 15 minutes more." This kind of reward system is always preferable to one in which the child loses something because it's more motivational - you're giving your child an incentive to do better.