After reading Nicole Cozma's post about
1. Power button for the lock
When I've been notified that I've just pocket-dialed someone, it typically happens right after I hang up on a call and have the phone app open when I slip my iPhone back in my pocket. A lucky somebody from my Recents or Favorites list gets dialed before the iPhone locks itself. To prevent pocket-dialing a contact after a call, hit the power button to lock the iPhone. Get in the habit of locking your iPhone before putting it in your pocket, and your pocket-dials will decrease if not cease.
2. Voice-mail screen is your friend
Tapping the voice-mail icon in the lower-right corner of the phone app should do nearly the same thing as locking the phone as a preventative measure against pocket-dialing. It's not quite as effective because your butt could somehow return you to the Recents or the Favorites screen--where a call is just a tap away--or call back someone who recently left you a voice mail before the phone locks. The odds are, however, that at most your butt will listen to a voice mail or two.
3. Passcode Lock to the rescue
If you find that you are pocket-dialing not after recent phone calls but at seemingly random intervals, try the iPhone's Passcode Lock. Go into Settings, and under General you'll see Passcode Lock. Choose a four-digit code, which must be entered before you or your butt can unlock your iPhone.