Here's the thing about ideas: when I'm deep into a project, the ideas materialize like popcorn. Not ideas about my project, mind you, but ideas for other things I should be writing. I wish I were more prolific, able to flesh them out before they fade away or become less interesting, but I'm a slow writer.
I love what Ray Bradbury has to say about ideas: 'If you stuff yourself full of poems, essays, plays, stories, novels, films, comic strips, magazines, music, you automatically explode every morning like Old Faithful. I have never had a dry spell in my life, mainly because I feed myself well, to the point of bursting. I wake early and hear my morning voices leaping around in my head like jumping beans. I get out of bed quickly, to trap them before they escape.'
Along with reading, a writer benefits from experiences as well. Try new foods, visit out-of-the-way places (museums, national parks, country backroads instead of the interstate), or take a class in something out of your comfort level (bellydancing! hypnotherapy! medieval combat!). An idea for your next main character or setting may be as close as your morning newspaper or the community education catalog.
Keep your eyes open and the pens and laptops ready!