The single most effective motivational technique I picked up with the candle method. It is so good, I think, because it focuses your child on the positive. No negative reinforcement required.
First get a birthday candle and a gift bag. Fill the gift bag with a small trinket or a coupon for special time you make. The key is you do not let your child look in the gift bag. Not until the candle burns out. This how the excitement (and motivation) builds.
When it is time to work on a school subject, you light the candle. As soon as your child looks away and begins to daydream, or starts talking to you about things not directly related to the subject, you blow the candle out. No warnings, just blow it out. Now, your child knows full well that he or she is not ever going to find out what is in the gift bag until that candle burns down. So instantly, you have gained their attention.
Begin with a birthday candle and then gradually increase the candle size so that your child earns the contents of the bag once a day, to begin with. Then you can gradually increase the candle size so that it only burns out once a week and you can make the prize in the gift bag a little more substantial.
This method works like an absolute charm, but only if you keep total control of the gift bag so it cannot be opened early. The bag also has to sit on your child’s desk while they work so they can see both the candle measuring time as they focus, as well as the prize they will get at the end. It helps if the bag is cool looking and glittery (is that a word?).
I heard about this method from another homeschooling mom, and have no idea who thought it up but it really is the best tool I have come across yet to gently help your child to learn to focus. If it worked for us, it will work for anyone. And its a great way to dole out all the little toys you have collected through the years from attending and giving countless birthday parties.