“I’m sorry this letter is so long, I didn’t have time to make it shorter.”
That's a quote from George Bernard Shaw, and it's one of my favorites. I've found myself bringing it up a in several recent meetings with clients.
It always gets a few laughs (or maybe they’re just being polite), but there’s a legitimate point here. It’s a lot harder to say less than it is to say more—at least if you want it to be any good.