How to increase your chances of survival in a plane crash?
« Some of the survival game involves factors passengers largely can't
  control, like the weather, flight crew skills, the design of airline
  seats and the construction, maintenance and age of the plane. But
  passengers themselves can do a lot to improve their chances of
  survival simply by making smart choices and being informed. »⁴


Things you can do:

Preventive Measures


Book the safest seat.
Avoid unsafe airlines.
Fly during daytime (as statistically it slightly increases chances of survival and some kind of incidents are far more common at night).


Preparatory Measures


Bring helpful items on board.
Clothes – what you choose to wear can help or hinder your escape:


Opt for flat, sturdy shoes. Flip-flops and high heels won't help you make it to the emergency exit.²
In case of a fire or glass and metal debris, long-sleeved shirts and heavy cotton trousers will offer more protection than shorts and a T-shirt.²
Form fitting clothes are less prone to catch fire.
Polyester and other synthetic fabrics will melt to your skin.³



Before Take-Off


Pay attention to safety advice. It can save your life. Read the safety card (in the seatback in front of you) as planes do differ.
Memorize the emergency exit locations before take-off. Plan and visualize your escape route. Count the rows (armrests⁴) to your nearest exit (in front of and behind you in case one direction is blocked!). That way, in the event of smoke filling the cabin or if emergency lighting fails and it’s very dark, you will know the number of seat rows and can feel your way to an exit.¹ Many passengers survive the initial impact but are killed by the ensuing fire, smoke and sometimes water. 90 seconds should be used as a time frame for escaping.⁴ 
Memorize where your nearest cabin crew are, because their help and instructions will make a huge difference.²
Be aware that at 35000ft you have just 30-60 seconds to don the oxygen mask and tug on the line to get the flow going. After that, you lose consciousness.²


In-Flight


Don't drink alcohol.
Seatbelt:


Keep your seatbelt on whenever seated (also prevents turbulence deaths).
Fasten it tightly - every inch of slack increase the potential g-forces you could be subjected to during an impact.²
Wear it across your pelvis, which will handle impact better than your stomach².
Practice opening and closing it:


  After interviewing 1,900 survivors and 155 cabin-crew members,
  Professor Galea made an interesting discovery: Most passengers lose
  valuable time because they struggle to undo their seatbelts. “People
  tend to try and press a button on the seatbelt, because in this
  emergency situation they revert to normal behaviour [learned from] car
  seatbelts [with] a push-button system.”²




During Take-Off and Landing (when ~80% of accidents occur²):


Don't sleep/nap wearing eye mask and earphones.²
Stay alert and aware of your surroundings.²
Keep your shoes on.²


When a Crash is Imminent


Brace for impact:



Put your feet flat on the floor, farther back than your knees ("aft position").²
Consider placing hand luggage under the seat in front of you to act as a cushion (to prevent your feet from hitting the seat, I assume). That way you are less likely to suffer broken ankles that will hinder your escape.²



Escape and Aftermath


After the initial impact it's common for fire to break out. Just a few breaths of toxic smoke can cause you to pass out. In the event of smoke/fire:


Cover your mouth and nose with a wet cloth (e.g. wet the seat back headrest).²
Stay low. (That doesn't necessarily mean crawl on your hands and knees. Although there will be less smoke at floor level, you could be crushed or suffocated under luggage and other passengers.) Instead, keep your head down.²

Don't retrieve your luggage.¹
Don't inflate your life jacket inside the plane. It hinders your escape.²
Move fast but resist the urge to push and shove. It's likely to make matters worse and cause others to lash out.²
Once on the ground get away as fast and as far as you can.²




sources: 1, 2, 3, 4