Friends moving away what do i do?

my sister has a friend at our martial arts club and her family is moving away because her mom got a job promotion i am friends with them to but not as well as my sister, but anyway what can i do to help my sister and her stay intouch or stay close to each other even after they move ? because my sister and her dont want to lose there friend ship...i dont want answers on how to keep them to stay here but just on how to make it easier on us....they are moving far enuf away we might never see them again.