This list comes from Coach Don Meyer who had done an incredible job of developing assistant coaches who have moved on and had great careers throughout the nation.
#1 Allow some responsibilities and opportunities to talk and teach.
#2 Track their progress and give feedback.
#3 At the start of practice, give each assistant coach some duties to perform.
#4 Ask your assistants for their thoughts and suggestions.
#5 Give them books/articles to read to help their development
#6 Supplement their pay through summer camps