A great confidence booster: Let them play the teacher and you play the student. Have them teach it back to you.
This student was worried that she still didn't know her x6, x7, and x8 multiplication facts and that she wouldn't be able to do them in class with another teacher that didn't know the strategies we've worked on.
I was confident that she knew them, but she wasn't confident yet. It wasn't until I had her be the teacher and teach them back to me (which she did flawlessly), that she could see that she was the one who knew the strategies. She was the one with the knowledge. She was the one with the power to use it whenever she needed it. She didn't need a teacher or a parent or a calculator. She was able to self-check and be confident in her answers.
I just love helping students figure that out!