Use the power of the camera/phone to motivate your child.

Most of us want to be fully prepared if we know we we'll be recorded.  Let your student know that you'll be recording the piece/measure/one line to share with Mom/Dad at work, Grandma/Grandpa, etc.  They will no doubt want to practice a bit harder once they see the first take! Hopefully the receiver of the recording will make a big deal about the effort and progress they observe. FaceTime or Skype are great tool as well.
To get your full value out of the camera/phone tool, keep the recordings and show them to the student from time to time so they can see how much they've improved - a very powerful reinforcer!
Remember:  We applaud the process as well as the final product!