Form and your core.
They both go hand in hand, a weak core will inevitable lead to poor form which as distances increase will lead to injury. The off season is the ideal time to get your core work in and remember your core is not just your abs it's everything from your neck to your ass front and back.
On form we would find it strange to start a swim workout before doing drills and working on technique, yet when it comes to cycling and running we give it very little time if any at all. The reason east and north african runners are so far ahead is because of their economy, american and european runners have on average a higher VO2 max but are not near as economical. Running drills need only be done twice a week before or after a workout and can make a hugh difference. Towards the end of a race something as simple as thinking about form can take your mind off the pain and help maintain your pace.
Drills
http://www.youtube.com/watch?v=vcH97Dx8VCk
Form from head to toe
Head should be looking ahead not down at ground
Chest out - think Johnson
Shoulders relaxed and down - once you get tired they tend to hunch up and restrict breathing
Arms should not cross the body and elbows should stay behind torso - should move back and forward think thomas the tank engine
Lean slightly forward - let gravity help out -not from hip your whole body
Keep feet below the torso
Midfoot or forefoot hitting the ground not heel - this acts as a brake and causes most injuries
Make sure your using your energy to move forward not up and forward, thats bounding not running - Bannister trained beside a 6 foot wall with a friend on the other side looking to see if his head popped over and he turned out to be fairly decent
When running downhill never ever increase stride length instead increase turnover and lean forward - your quads will thank you
Perfect form except for her arms as some people have commented on - note most of us are slightly taller and heavier
http://www.youtube.com/watch?v=Pd1QvXJS ... re=related
If that all seems like too much bother there is a simpler way to get faster. Forgo the christmas dinner and get on a diet EVERY POUND OF BODY WEIGHT LOST IS WORTH 2 SECONDS A MILE. Thats 28 seconds a stone. I'm around 11.5 so if I can just get myself close to the 7 stone mark I reckon I can win the 1500m 5000m double in London.