If you realize an emergency landing is imminent, you should immediately adopt the brace position.

In this case, the emergency landing was a prepared one - flight attendants would have been able to brief passengers on their nearest exit, removing eyeglasses, pens, sharp objects from their person , stowing all objects that could become projectiles. Part of the prepared emergency landing briefing is to have all passengers practice the brace position before adopting it about 20 seconds before touchdown. Typically the flight attendants will shout "Brace Brace" or "Bend over, grab your ankles".

However, in an unanticipated emergency - such as a rejected takeoff, runway overrun or other sudden emergency - you may not get any warning from the crew. Properly bracing can dramatically decrease your risk of head injury (i.e. avoid you from becoming unconscious post-crash!) and limb injury. Generally in a forward facing seat, you want to position your head on whatever you are most likely to strike (ie place your forehead on the seatback in front of you if you can reach it). It is best to check the aircraft safety card to see what is the airline's recommended brace position (it's required to be on the card.)

Remember, in the US commercial passenger aircraft have a "16 G Survival rule"...you're supposed to be able to walk away from a 16G crash, and thus seats are actually designed to be energy absorbing. Getting in the brace position enhances your survival chances significantly.