I say this a lot to my students – every day!
You've gotta have enough space between you and the piano or keyboard in order to maintain good playing posture.
Here are three reasons why students aren't sitting back far enough from the piano, and some solution tips:
You are trying too hard to read the music. – Just like in school where you need to be able to see the blackboard, take any necessary steps to assure that you can see the music when it's up on the music stand. If you wear glasses, bring them to your piano lessons.
You are wriggly, and can't sit still. – Be sure to use the bathroom, eat a light snack, stretch your arms and legs, shake out your hands, and make yourself comfortable before playing the piano. This will help you to sit still and focus during your lessons.
You didn't think to get adjusted before sitting down. – Always push the piano bench in to, or pull it out from, the piano or keyboard so that, when you sit, your arms are not too straight and not too bent.
Correct posture, as mentioned before in "Keep your hands tall", allows for nimble technique and can help prevent injury.