Also, just to add to the above, when adopting a brace position, the following is usually desirable. There are many variations of brace positions (the FAA and UK CAA recommend slightly different positions, to name one difference), but there is some commonality in all brace positions.
1) Pre-position your head, if possible, on the object you are most likely to strike with your head. (Usually this is the seat-back in front of you.)
2) Avoid leg flailing by adopting a "feet back" position. (Feet flat on the floor, slightly behind the knees)
3) Lean over to some degree. If you can only lean over and rest your head on the seat back, that's fine. If you can lean all the way over and grab your ankles, that's fine too.
4) Prevent arm flailing. The UK CAA recommends hands on top of another (not interlocked) and placed around the head. The US FAA recommends either wrapping your arms around your legs, or, resting your forehead on crossed arms, with the arms against the seat back.
Things to avoid:
-Having your head to one side
-Having any part of your limbs outstretched and locked against an object
-Being in an upright position UNLESS your aircraft seat is equipped with a seat-belt mounted airbag. (Found of Virgin Atlantic, Air Canada, Singapore Airlines and Air New Zealand premium aircraft seating, to name a few) or you are wearing a shoulder belt or harness.