As anyone who has ever applied to the NSF for a grant knows, such a proposal is a slightly odd piece of writing, not quite like anything else mathematicians are called upon to write. As such, it's a hard thing to learn to do well; of course, the basic requirements are set out in the grant proposal guide, and explained a bit more clearly in some other sources. But there's no substitute for reading an actual specimen.
Now, generally this is achieved by asking some older colleague to see an old proposal, an approach that works just fine in most cases. On the other hand, if there are any publicly available old NSF proposals online, I think finding them would be a boon to all of us preparing proposals, if only to have more data points. Does any one know of any (I didn't have much luck on Google).
I'll just note: I'm aware that the NSF has abstracts of all funded proposals on their website, which is useful, but doesn't give much insight into, say, what people write about broader impacts.