Motivation and morale go hand in hand. If you find yourself unmotivated, take a break and step away. As has been said before, you can't be motivated 24/7. It's really just not possible.
If your trying to power through and get a project done, I find it's best not to focus on the project as a whole but as each part unto itself. It stops the mountain of unfinished work from getting to you. Reading motivational articles can help, but if you're simply to fried to read, set it aside and go do something else for a little bit. It's easier to step away then it is to wait until you're not to fried to work.
I like this blog in particular, and I have this specific post bookmarked for when I'm going through a lax period. I've found if I can't do, reading some production blogs can sometimes give me the motivation to continue learning(even if I don't learn anything). Some of them are quite funny, and you can usually glean at least a little useful information, even if you've been working on bitswitching until your eyes have bled.