We've yet to try this, but according to a writer for The Next Web, if you experience a loss of service on your iPhone, you can regain some bars by dropping it in an empty glass

According to the writer, he was in a restaurant with horrible reception and the waiter told him that if he wanted better service to place his iPhone in a glass. Though in complete disbelief, he did as instructed and saw his bars jump from zero to three. 

Sounds like b.s. to us, but give it a shot. Read his whole account here

[Business Insider via The Next Web