There's a list of GRFP proposals you can browse here. My essays are here.
From my small sample size of submitting twice, my main advice for the proposal is: The proposal is largely a credibility game, and one of the best ways to signal credibility is to be specific and show that you've thought things through (eg in my proposal, I showed off in a parenthetical that I knew about the equipment I'd be using down to the centimeter). This is kind of silly, but my gut feeling is that it goes a surprisingly long way. My first proposal was rejected in part because I hedged my bets about what my study species would be because I didn't know where I'd end up doing my PhD. No one is going to force you to follow your proposed research exactly (or at all; my dissertation ended up being completely unrelated to what I proposed), so picking one specific path can't hurt you.