To engrave on a locket with a photo of my father after his death
To remove potential ambiguities you could try:
"numquam memoria excides." =
(literally) "You will never fall out of memory." = "You will never be forgotten."
Alternatively, using the infinitive of "excido":
"numquam memoria excidere." =
"Never to be forgotten."
(An active verb translated passively.)
'Remembered' is a past particle. One way to translate 'to remember' is 'memorare'. I would go with a particle perfect passive, as it is a person who is being remembered, but I'm not really sure about this. Maybe someone else can give a better alternative for the verb form.
So the translation would then be: