While this is probably not a feasible solution for you to actually implement due to any league rules that you may be playing with, the safest solution could be to remove helmets from the game altogether.
For a different take on the issue, the Wall Street Journal had an article in 2009 that brought up the possibility that the presence of the helmet in football is a major contributor to concussions in the first place and removing the helmet altogether could help reduce the number of concussions in the sport.
The article notes that a similarly rough sport, Australian Rules Football, that no helmets are used and the probability of getting a concussion in the NFL is 25% higher than in the AFL (Australian Football League). The reasons for this are that players in the NFL often feel like the helmet makes them "invincible" and they tend to turn their bodies into projectiles, using their head as a weapon.
Also, it seems that recent studies done on players has shown that long-term brain damage is not caused by big hits to the head, but by the accumulation of thousands of smaller hits which no helmet could ever prevent. Currently the organization that rates helmet safety, NOCSAE must give a helmet a "severity index" of 1200 or less for the helmet to be eligible to be sold. It is speculated that to prevent the risk of concussion, a helmet would have to have a rating of 300 or less.
So, in effect, current football helmets would have to be four times safer than they currently are in order to prevent concussions. There are no efforts being made currently to ban football helmets at any level of American Football.
