So you're one of the fortunate (if not rabid) few who've found themselves the proud new owners of an Apple iPhone 4G. It's Dieter Rams inspired housing incorporates separate wi-fi/Bluetooth/GPS and cell antennas as two separated elements all around the case, a design decision which at first seems perfectly Apple in execution (except for the darn visible seam). That is until you notice "holding it wrong" drops reception bars faster than pants at a nudist beach. Don't fret, there are several ways to improve your iPhone's reception and patch over the danger zone.
Get the iPhone 4 Bumper: the rubber and molded plastic seems to hint Apple may have known beforehand about the antenna issue. The $29 accessory seems overpriced (and Apple has made it clear they have no plans of offering them for free), but user feedback has verified the accessory reduces, if not rids, the bar reduction/death grip issue. The bumper also allows the iPhone to placed face down on hard surfaces without worry of a scratch, thanks to the raised edge it provides, but the quality and fit seems to be an issue (alongside the colour selection)
Hold the phone THIS way!: Steve Jobs has replied to iPhone owner's complaints with a mildly dismissive (though completely Jobsian), ""Just avoid holding it in that way." The official Apple advice is to avoid gripping the iPhone 4G in the lower left corner in a way that covers both sides of the black strip in the metal band.
As reported by AnandTech's excellent in-depth review of the iPhone 4G, the phone's reception is affected by "making skin contact between the two antennas does result in a measurable difference in cellular reception" because of the lack of insulation on the surface of the antenna to prevent impedance via the user's body/hand. For those looking to forgo adding any additional accessory onto their phone, this may require some readjustment of how the phone is held while in use. Annoying and troublesome from an ergonomic perspective.
Isolate or Move the microSIM Card: we're putting this tip under the "try with caution" category, but some MacRumors readers are debating the effectiveness of a possible fix where moving SIMs, covering them with electrical tape or trimming the size of the card to isolate it from the tray may reduce the antenna issue by a bar or two (some are reporting complete fixes).
The Budget Solution - Clear Scotch Tape and Live Strong Rubber Bands: a notable low cost discovery noted by a few iPhone 4G users is scotch tape applied along the area where the seam is visible works well at improving reception issues. A $1 LIve Strong Lance Armstrong wrist band can also be stretched to basically do what the pricier Apple branded bumper offers. Just be careful while stretching it and putting it onto your phone, lest you slingshot your problem away and create a whole new one.
3M Vinyl Stickers for Death Grip Corner: we actually thought of this immediately upon reading about the "death grip" issue; we've applied similar protective vinyl appliques for non-scratch protective purposes onto our laptop and iPod Touch, and we thought some of the spare pieces would work well at preventing body interference between the two antennas. Apparently so did someone else, selling a $10 strip of clear vinyl.
Personally, as someone who doesn't own a cell phone, but whose better half is up for a phone upgrade (iPhone 3G), we're advising holding off on purchasing the iPhone 4G due to this major issue. Bumpers, scotch tape or particular awkward holding positions does not make for an excellent design, and at the premium Apple is selling these phones (and AT&T chaining subscribers, problems and all, to two years), we only hope Apple takes care of the issue with a software upgrade, alongside a rebate for 1st adopters who will have to learn to live with the issue.