From A List Apart:
Ah, that magical moment when Moleskines reach capacity, people pass out from dry-erase fumes, and there are no bad ideas (except for that one…). The time-honored brainstorming session (even confined to one brain), helps you build design concepts around strategy. No reason your copy can’t come along for the ride.
While you’re sketching designs, jot down a quote or two. Collect tear sheets of words as well as images. Shoot rough video of someone you think would make the perfect spokesperson. Remember that by introducing your narrator persona, you’re creating an expert peer your users will come back to for advice, information, and inspiration. That’s worth spending some time on. It also makes the actual business of copywriting much easier. Learn the language, then tell your story—not the other way around.