The Canadians have done the most C&R research. Here's a link to a review of guidelines, based on research, from the Ontario Canada Ministry of NR:

http://www.mnr.gov.on.ca/stdprodconsume/groups/lr/@mnr/@letsfish/documents/document/228125.pdf

And this is the study upon which time out of the water was numerically linked with death in rainbow trout:

http://www.cnr.uidaho.edu/fish511/Readings/Ferguson%20and%20Tufts%201992.pdf

This is from the Ontario C&R guidelines:

Handling and Photographing a Fish
• Keep fish in the water as much as possible to minimize air exposure.
• Never place your fingers through gills or in the eyes.
• Don’t hold heavy fish by the jaw as this may damage the jaw and vertebrae.
• Hold large fish horizontally and support its body to avoid damage to the internal organs.
• Use wet hands or wet cloth gloves to handle the fish.
• Have camera ready prior to landing fish to minimize air exposure.
• If possible, photograph the fish while in water.