It helps to plan something at a certain time, even just going to dinner or on walk. Even if I'm totally engrossed in whatever new project I'm doing, I need to leave. I can keep thinking about it and stay excited, while not burning out on it.
I also try to find things I can work on a little bit every day for a longer period of time. If I want to learn a foreign language or the fancy programming framework of the month, I do much better if it's used every day for 15 minutes than all at once and then forgotten.