Whether it's keeping a journal handy to jot down a "trend" you've seen multiple times, or keeping a Tumblr with inspirational photos for you to look back on, documenting the things that keep you interested is a key. Actively searching for things to write about is fine, but you shouldn't have to do it all the time. You should have a hundred ideas bouncing around in your brain at any given moment, and if you run out, replenish the supply by reading the paper, browsing your RSS feed, or lurking around your favorite bar or coffee shop. Nothing you encounter is exempt from your discerning eye as a writer.