"Opportunities don't often come along, so, when they do, you have to grab them"
-Audrey Hepburn
That's one of the two I use most, from one of my fave people in the world, the beautiful Audrey.
And an encouraging one I use is "Look to the Sky" because, simply putting it, when you look upwards, you show your pride no matter the circumstance.

I also love these two:
"When I was younger, I would remember anything, whether it had happened or not"
-Mark Twain
I just find it both sad and intriguing how imagination changes as we grow older.

"Everything has it's beauty, but not everyone sees it"
-Confucius
Because everything we say isn't beautiful is sometimes more beautiful than the things which are labeled. To me, everything has beauty.