On less than 24 hours' notice? Not much more than "Duck!". The best we could have done was wait until an accurate trajectory was calculated and then, if it happened to be headed for a populated area, tell them to run like hell. Or, if it was to land in the ocean, to evacuate coastal cities to whatever extent possible.
Which probably wouldn't be much. Basically you'd just have mass panic and gridlock.
And, of course, a 500-meter asteroid would be a hell of a lot worse than a 30-meter one.
On less than 24 hours' notice? Not much more than "Duck!".
"Duck" works.
Several other things also come to mind:
"Repent! The end is near!"
and
"Kiss your ass goodbye."
As an aside, this line from the Toho Japanese monster movies never seemed to do much good:
"Remain calm. Leave the town in an orderly fashion. Don't panic!"