友だちを大切にする。ともだちをたいせつにする。tomodachiwo taisetsuni suru.

As in a farewell type of greeting? I tend to say "deha, tassha de" では、達者で which essentially means "take care of yourself". You could also say genki de ne 元気でね, but I believe that たっしゃで shows more maturity in a certain way.