Quote Originally Posted by Red Lioness
In all the places I've checked, MacBeth Clan has no listed motto. Oh well. . . .
From Electronic Scotland and the Clan MacBeth Society of North America:
Motto: Conjuncta virtuti fortuna.
Translation: Fortune joined to bravery.

As for me, the Robertsons and MacDonalds have already been mentioned, so:
Stewart - "Virescit vulnere virtus"
"Courage grows strong at a wound."

And Sackett - "Aut nunquam tentés, aut perfice."
"Do or do not, there is no try." (Not an exact translation, I know, but it expresses the meaning in a culturely familiar way. )