Here's the quote I keep close to my heart.
Tell me and I forget, teach me and I may remember, involve me and I learn. - Benjamin Franklin
Here's some other quotes that I really like, too!
"It's hard enough to find an error in your code when you're looking for it. It's even harder when you've assumed your code is error free." – Steve McConnel
"If debugging is the process of removing software bugs, then programming must be the process of putting them in." – Edsger Dijkstra
"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." – Brian Kernighan
"Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live." – Rick Osborne
"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." – Martin Fowler
"Copy and paste is a design error." – David Parnas
"Any code of your own that you haven't looked at for six or more months, might as well have been written by someone else." – Eagleson's Law