Like you Brian, I am generally motivated by the desire to learn new stuff. The challenge of mastering something new keeps me obsessively motivated until I feel like I have completed that challenge. e,g. Moving from Arduino to ATtiny AVR to ARM keeps me engaged.
Completing projects has been a useful motivator especially when I need to learn new skills. e.g. An LED lighting controller for a room in our house kept me busy for a while. I started with a relay and radio control; upgraded to ATtiny PWM drive of a MOSFET; added IR and ZigBee control; replaced the Buck convertor with one I made and then integrated everything into a single SMT PCB. However I still find it a challenge to find the next engaging project.
Another thing that helps me is maintaining a blog BecomingMaker.com. Consolidating what I have learned in a form that I hope others will find useful helps keep me motivated. It also forces me to dig a little deeper into the details and clarify my thinking.
Setting aside one evening a week has been helpful to keep things moving when life gets busy. Often getting started on the next step is half the challenge and having some time set aside can get me over the hump when some energy is required to get things moving again,
Also buying a new component or dev board can help. We all enjoy playing with new toys!