Acknowledge Mile-Stones in Teens' Lives

[I found this picture of smooth stones free at  Dreamstime.com ]

Get a collections of stones - paint 'Mile' on the bottom - when one of your teens achieves something - passes an exam, reaches a birthday, graduates from middle school - write the date on it, and what the 'milestone' is, and give it to them.  It's the sort of thing they'll keep for many years, and it will remind them that you are genuinely interested in their life.