Like many other cell phones, your iPhone allows you to assign individual ringtones to your contacts. This provides audio clues as to who is calling without having to look at the iPhone screen. To make this happen, tap the Edit button at the top right of any individual contact screen and scroll down until you see Assign Ringtone and tap it. Select a ringtone and tap Info to return to the Contact info screen.
If you want to have a bit of fun and kick the trick up a notch, record individual messages by the people themselves. Use your favorite mp3 recording program at your computer and use a ringtone installer such as iFuntastic for the Mac or iPhoneRingtoneMaker for Windows to add the personal ringtone to your iPhone. "This is your mother, John. Pick the phone up Right Now." Hee!