Redlines and Deadlines has a nice couple of articles on building a world 'bible' (1 and 2. Personally, I find prompt tables help. Because you're not constrained by plot, or even characters, you can explore bits of your world without worrying about how it effects the novel. You discover random facts (for example, my fantasy world has very little glass, and sees exact timekeeping as the province of the wealthy, thanks to a couple of prompts) that you can work in without exposition, because they came to you as naturally as they'll come to your reader.