How do you guys get motivation at work?

I really like what I do as a designer, but sometimes I get stuck trying to get things done without success. I either have no tasks left and it takes to much of my to write new tasks, or the task at hand isn't as straightforward as I thought and it is too difficult to keep going and so I start procrastinating more and more and when I look at my current progress, it's been a while and I've done practically nothing. I would like to hear if there's someone else like me and how do you cope with it.