Very often people call you, and very often you don't want to answer. A great way to stop the ringing of the phone and not hang up on the person is to send the call directly to voicemail. This is an extremely simple and easy iPhone trick.
How do you do it?
Double click the Sleep/Wake button on the top right of your iPhone.
If you are using headphones, simply double click the microphone button. The only downside to this trick is, well, listening to people's voicemails.