Most iPhone users have probably experienced the annoyance, or even occasional embarrassment, of their cheek hitting the iPhone's "end call" button and accidentally hanging up on the person they're talking to.
Depending on what model iPhone you have, there's an easy way to avoid this.
If you have the original iPhone, iPhone 3G, or iPhone 3GS:
When your call has started and you're talking to someone, just press the iPhone's hold button at the top right of the phone. This will lock your phone but will not end your call.
Now, your cheek can't end your call. When you're ready to hand up, just unlock the screen by using the onscreen unlock slider and tap "end call." Problem solved.
If you have the iPhone 4:
Unfortunately, Apple changed this functionality with the iPhone 4. Now, if you're on a call and press the lock button, you'll hang up. There are, however, two exceptions to this: speakerphone or headsets/earpieces. If you're using those functions, the button still locks the screen rather than ending the call.