Another is to put a black dot on three out of 12 wiffle balls and instruct the hitter not to swing at any ball with a black dot on it. I haven't tried this personnally but have heared it works.
Concentration is the key.
Simple things seem to work the best. They love it and you can kneel and throw pretty hard from 25-30 feet. These kids are amazing at times. Have the best of seasons. Dry and warm by normal standards.