If you are on the edge of reception, these steps will help..

1. Turn off 3G – like I have not said that enough.
2. Put the phone down.  Holding the phone reducing reception.  Use handsfree or a headset. Wired headset preferably.
3. Plug it in.  The iphone works better both because the cable is attached and the extra voltage that the direct power supplies.

Enjoy!