Hi Klaas,
I always believe that motivation comes from within. And to do that, you need to really understand what you want to achieve.
For me, I always wanted to have more time for myself, my family and my love one.
So, the next time you are feeling down or didn't perform according to what you want, think of what you want to achieve in life and why.
By the way, reading quotes, bibliography, motivational books, blog posts and articles do help.
Here are some sites you can visit for motivation:
Motivational Articles: A Collection of Motivational Articles
Motivation Articles, Essays, Tips and Advice
Self Improvement
Motivational Articles
Another easy and fast way to get motivated is to talk to the Forum Members.
Hope that helps.