Here's a few off the top of my head:
Similar to what RUNS posted - 'Be Nice - Until it's time NOT to be Nice.' (what movie was that from? I can't remember)
Story retold by Bruce Lee in TofJKD:
A learned man once went to a Zen teacher to inquire about Zen. As the Zen teacher explained, the learned man would frequently interrupt him with remarks like, "Oh, yes, we have that too...." and so on.
Finally the Zen teacher stopped talking and began to serve tea to the learned man. He poured the cup full, and then kept pouring until the cup overflowed.
"Enough!" the learned man once more interrupted. "No more can go into the cup!"
"Indeed, I see," answered the Zen teacher. "If you do not first empty the cup, how can you taste my cup of tea?"
Isaac Newton:
If I have seen further than others, it is by standing upon the shoulders of giants.
Einstein:
"Great spirits have often encountered violent opposition from weak minds."
"If A is a success in life, then A equals x plus y plus z. Work is x; y is play; and z is keeping your mouth shut."
Another LE saying:
Deny everything, demand proof, and make counter-accusations...
Heard on the PT field:
Pain is just weakness leaving your body... Sandbag on your OWN time!
In a fortune cookie:
The greatest pleasure in life is doing what people say you can not do.
(I took it to mean what people say you are not capable of doing...)
Through personal experience:
There is ALWAYS someone better or stronger or faster or YOUNGER...
Gallagher the comedian:
What's the opposite of Progress? Congress...
Curly of the Three Stooges:
If at first you don't succeed, keep on sucking until you DO succeed...
From my dad:
Live within your means.
Don't get smart with me.