One of the nation's leading researchers in this field, Dr. Stefan Duma says, "My opinion is they come off because some players want them too, smile for camera etc. so they wear it loose." If Dr. Duma is correct, then no amount of research by the manufacturers and no amount of rules changes will improve the situation.
The top three helmet manufacturers in the United States are Riddell, Schutt, and Adams (formerly Bike). Since the early 70s and after a few high profile cases of head injury and concussions, much of the research has been focused on designing head gear that will reduce the chance of and/or prevent concussions. Many technological advances have been made. In 2002, Riddell introduced their "Revolution" helmet and in 2007, Schutt added the line of "Ion 4-D" products which they tout as "the most advanced helmet in the market". Adams' premier product is the "A-4".
Perhaps chin strap design is or should be the next area of significant research. A source in the NOCSAE (National Operating Committee on Standards for Athletic Equipment) reports that one area of research involves the loosening of the chin strap on impact caused by the compression of the interior padding. Such a change inside the helmet would result in easier removal. Combine this phenomena with Dr. Stefan's theory and you've got a recipe for disaster. A player with no helmet is the most vulnerable for head injury.
The February 2007 issue of Popular Mechanics reports that extreme football collisions can measure up to 150 g's compared to a roller coaster's five. Player safety depends upon the equipment to distribute the incoming energy of impact to lessen the severity. Typically discussions involving player safety and indeed, much of the research and development by makers of the equipment, focus on helmets and shoulder pads. The article referenced in Popular Mechanics is similar although it does mention the nagging frequency of knee injuries.