You Are What You Eat: if you eat healthful foods you'll be healthy, if you eat unhealthful foods you'll be unhealthy.
Example: To keep healthy, it’s best to keep off junk food. You are what you eat.
You can't Judge a Book by its Cover: don't prejudge the worth or value of something by it's appearance alone
Example: The candidate did not look very intelligent, but we can’t judge a book by its cover.
You can't take it with you: cannot take your wealth with you when you die, you ought to enjoy it while you're alive.
Example: When you die, you can't take anything from this life with you.
Your guess is as good as mine: to have no idea, do not know the answer to a question
Example: How long do you think we should have to wait for the bus? Your guess is as good as mine.