Summarize this article:

Waterproof cases form an airtight seal around your device. A change in altitude or temperature can cause a pressure change that makes removal much more difficult. If this happens, equalize pressure by inserting a coin into the gap next to the latch on front of the case, then twisting the coin. Large models have a "purge knob" on the front of the case instead, which you can turn to open and equalize pressure. Tighten the purge knob before using the case again. The case is not waterproof while the knob is open. Some Otterbox cases, such as the Armor series models for the iPod touch, do not separate into two parts. Instead, they open when you rotate a latch on the back of the case. The case will pop open enough to remove the phone, but the two halves remain attached at one end.

Summary:
Equalize pressure on waterproof models. Open hinged Armor cases.