Listen to music. In the old days, people used to listen to classical music -- this was true of Proust and Thackeray's "Vanity Fair" through the Pynchon of "Gravity's Rainbow."
Nowadays, it has to do with alternative music, and thankfully, there's a lot of bodies of work to draw off -- replacing the old canon. I can't imagine having good ideas without listening to Dinosaur Jr. and Sonic Youth.