Knock on wood
Definition 1 of 1
Pro Tip 1/3Superstitious Roots
Originated from ancient beliefs that spirits live in trees and can be called upon for protection. 
My interview went great, knock on wood!
Pro Tip 2/3Informal Use
Most appropriate for casual conversations, not in formal writing or professional settings. 
We've always had good weather for our annual picnic, knock on wood.
Pro Tip 3/3Express Sincerity
Shows you are not boasting and that you acknowledge good fortune can change. 
I haven't missed a day of work this year, knock on wood.