Here is another one of my favorites.
If you Think You Can, You Can!
You can be a total winner even if you're a beginner,
If you think you can, you can,
If you think you can, you can,
You can wear the gold medallion,
You can ride your own black stallion,
If you think you can, you can,
If you think you can, you can,
It's not your talent or the gift at birth,
It's not your bank book that determines worth,
And it isn't in the color of your skin,
It's your attitude that lets you win,
You can upset Connors or Austin,
Or win the marathon at Boston,
If you think you can, you can,
If you think you can, you can,
You can profit through inflation,
You can redirect this nation,
If you think you can, you can,
If you think you can, you can,
It doesn't matter if you've won before,
It makes no difference what's the half time score,
It's never over until the final gun,
So keep on trying and you'll find you've won,
You grab your dream and then believe it,
Go out and work and you'll achieve it,
If you think you can, you can,
If you think you can, you can!
~ Denis Waitley ~
One of my favourites: 
You are not enclosed within your bodies, nor confined to houses or fields...that which is in you dwells above the mountain and roves with the wind...it is a thing free, a spirit that envelopes the earth and moves in the other.
-- Khalil Gibran
Yeah, I keep telling my boss that.

Another of my favorites is from St. Francis of Assissi:
"Lord give me the strength to change that which I can, the serenity to accept that which I can't change, and the wisdom to know the difference."
I must not fear.
Fear is the mind-killer.
Fear is the little-death that brings total obliteration.
I will face my fear.
I will permit it to pass over me and through me.
And when it has gone past I will turn the inner eye to see its path.
Where the fear has gone there will be nothing.
Only I will remain.
-The Bene Gesserit litany against fear [Dune series]