It’s always a good idea to have something that the students can quickly learn, to grab their attention. There are many different attention grabbers out there (clapping, bells, sayings).
Whatever you use, try to be consistent so you don’t confuse the students!
Some of my favorites include:
Teacher says / Students say
Macaroni Cheese / Everybody Freeze
Pop! / Corn!
Class (in a funny voice) / yes (said in same funny voice)
Hands on top / That means stop (hands are on head)
What do you like to use?