One of Siri's lesser know features is that you can chat with her at any time simply by lifting the iPhone to your ear when the screen is on. This can be done from the lock screen, home screen, or from within an app. If the screen is on, it works. Just lift the phone to your ear -- as if you're about to take a call -- and you'll hear the Siri tone, meaning that she's listening for your command. (To make sure this feature is turned on, go to Settings > General > Siri > Raise to Speak.) This is especially nice when you're in a situation where you are not able to look at your phone.
Now here's the part that's new to me. If you do this same "Raise to Speak" motion when the keyboard is currently displayed, instead of invoking Siri, whatever you say will get transcribed into text.
The next time you find yourself typing a rather long message, just lift your phone to your ear and say what you'd like to type. Dictation takes care of the rest and types out your words. Nifty, right?