Steven Pressfield, author of Gates of Fire and the War of Art, has a weekly column on his website called "Writing Wednesdays." I've been following Pressfield for sometime now. In my opinion he's has some of the best advice on writing out there--a lot of which can be applied to gaming, especially for DMs.
Last Wednesday's column, "How I Get Ideas", Pressfield tells how ideas just sort of come out of nowhere, usually unexpectedly while the "brain is turned off."
I can see where he's coming from. Ideas pop in at the most inopportune times, for me at least. Even though I'm no longer running games I still get ideas for adventures and campaigns. Fortunately I usually keep something to write on nearby.
So where do you get your ideas for your games? Do you actively seek them out or do let them come unbidden?