"cross your fingers"
An old superstition says that if you cross your fingers (usually two fingers of the same hand) it will bring good luck. Example: "I don't know what I will get for my birthday, but I am crossing my fingers that it will be a bicycle."
You cross your fingers when you try to make something happen by wishing that it will happen. A common saying: "Let's cross our fingers and hope for the best." Example: "Good luck on your test tomorrow. We'll be crossing our fingers for you!" Reply: "Thanks."
The meaning of don't cross your fingers changes a bit. It means don't hope too much for what you want to happen, because there is a strong chance that it might not happen. It means that you should not have expectations that are too high. Example: "It would be great if our team won today, but don't cross your fingers." Example: "We are hoping that John will get accepted to medical school, but we are not crossing our fingers."