If you are the proud owner of a brand new iPhone 4 you could be one of many people who have discovered that if you hold it in your palm, [as you would of course,] the slight electrical charge of your skin creates a connection between the two antennae that wrap around the device, causing dropped connections.
Get about an inch of scotch tape off the reel (that's Sellotape if you're in the U.K.)
Cut it carefully so that you make a rectangle something over a half-inch long and just tall enough to exceed the iPhone 4's height (about 0.4 inches should do it)
Align the piece of tape so that it's covering a section of the lower left metal side of the iPhone 4, some below the thin black plastic line and some above
Stick it around the phone's metal edge, making sure that all of the back of the edge's metal strip is covered--you may even be able to leave the front uncovered, as it's the back where your hand will rest. The main thing is to ensure that it's unlikely that your palm will touch the metal on both sides of the black plastic line
BTW, in case you're wondering about the post title, we proudly use the English plural of the word antenna. You're welcome!