I think every developer faces the same problem at one point. What helped me personally was actually working less and find new hobbies (that don't involve computers), and one thing in particular: running. It's hard to keep your enthusiasm at a high level if all you see is work - despite the fact that you might enjoy it. Go out, meet your friends, eat well and excersize.

EDIT: Recently I was attending a startup meeting and there was this video, where one developer said Sleeping and eating are overrated. What a load of crap - following this advice is the best way to burn out. We are not robots.