Keep An Eye On:
Can you keep an eye on my baby brother while I talk on the phone?
Keep your chin up:
Stay positive when things get difficult.
Keep your chin up. Things will get better.
Kick The Bucket:
Don’t kick the bucket! I need you around!
Knock On Wood:
Literally knocking on wood (or other hard surface) for good luck.
When my friend went in for a university interview I said, “Knock on wood!”