I like this one! Might be a tad long, but you could use just a verse!
"Stretch yourself to the limit
See how far you can go
You may do wonders, you never know
Stretch yourself to the upmost
See how far you can move
First you'll make blunders, then you'll improve
Keep on working - keep on striving
Pretty soon you'll be arriving at the top
Just don't stop!
Stretch yourself to the limit
Aim your sights at the sky
You can work wonders and you can fly" - morning glory