A recent post at the TiPb blog runs down the many many causes of Wifi interference with your iPhone, more than the advice to hold it differently. Some of the items on the list may surprise you.
The article talks about some of the differences between the radios and frequencies supported by different iPhone models, potential environmental problems, and a range of obscure hardware and physical barriers that may cause a problem.
Even the responses offered tips:
I had a customer who we could not get a decent signal one room over from her router, stick the USB adapter out the door and the signal was 100 percent, in the room maybe 15 percent. After much testing I came to the conclusion the walls were painted with lead based paint (built in the 50’s) and that was causing the signal drop.
I can believe this.
In urban areas, there's also the potential for users to live near power sources such as transformers that could cause interference.
I recently visited an apartment that was in the shadow of a transformer — it was on a pole right just a few feet outside the living room window — and the occupants were unaware of it. It wasn't really in their primary view and for the persons living there, this very large piece of industrial equipment just faded into the urban landscape.