Bad Signal? Check your SIM Card
It might not seem very intuitive, but iPhone Atlas is reporting that if you're experiencing bad signal strength on your iPhone, you might want to consider checking that the SIM card is properly seated. They even go so far as to say putting a small piece of scotch tape on the back can help with the connection. I've personally experienced the benefits of using scotch tape on a SIM card on lesser phones, but I've never had a problem with the iPhone.
Other options for boosting signal strength via uncomfortable hacker include: Resetting your phone and even attaching the USB cable as an ad-hoc antenna (ugh).