Fresh out of the box, your iPhone 4S can launch Siri in one of two ways: You can press and hold the Home button, or you can simply lift your phone up to your ear, and Siri should start listening. An anonymous hints reader discovered that Siri’s Raise to Speak feature works for text-entry, too.
Whenever your iPhone 4S displays the on-screen keyboard, there’s a microphone icon at the bottom left; if you tap that icon, Siri will begin transcribing whatever you dictate. But you can save yourself that tap by lifting your iPhone to your ear when the keyboard appears on the screen. A single tone will sound (as opposed to Siri’s more traditional double-beep) to indicate that your 4S is listening and ready to transcribe. When you’re finished dictating, simply lower the phone again.
Remember that the Siri’s Raise to Speak option may contribute to battery drain. That’s a reasonable possibility, because the technology works by constantly checking the phone’s proximity sensor to see if an ear happens to be near. If your phone's battery life is suffering, you can disable the option in Settings -> General -> Siri—though then this hint won't work.