"Make good choices."
Edit: Just realized this is pretty similar to the "Choose well" listed below, although it DOES sound a lot less ominous. And I think it functions as a universally applicable catchphrase that it's okay to use reflexively.
A literary allusion to the classic Freaky Friday.
Is there any rationalist equivalent of "good luck"? I've tried a few variants, such as "work well", "knock them dead", "we're with you" and certain situation-specific phrasings, but haven't found anything that worked generally - though a hearty "may all the gods of Olympus be with you!" can serve. Not a vitally important point, but it would be nice to have something similarly supportive and yet accurate to say.