The helmets my kid's team uses are the top of the line for non-custom made helmets. They use a series of inflatable pouches that can be pumped up to fit a particular head.
The problem is that even the best helmets only prevent the skull from bashing into things -- they don't prevent the brain from bashing into the skull. Much like what you hear about shaken baby syndrome, it's the brain sloshing around and banging into the skull that does the damage. The damage happens every time the skull stops moving before the brain does, which is really every time there is a sudden impact. Over time even small repeated injuries add up to substantial damage.
No helmet will ever prevent it from happening.
There is some new technology coming up that might help. Most notably is a chin strap that measures the force of impact and calculates the time a player should sit out before rejoining the game -- IF they're allowed to rejoin the game at all.
There are also some new computer apps that a coach can make the player run through that measures cognitive ability following an impact. If a player can't pass certain field tests they aren't allowed back in.
Concussion treatment has changed dramatically over the last few years. Each year the coaches in my kid's league have to attend a seminar on concussion management so that they're aware of the latest research.