Utilizing your energy in the most efficient manner possible is the key part of speed progression. Simply put; the more fluid and graceful you are, the less oxygen you'll be using as you run.
As fitness improves and speed increases an inefficient run stride will become a major limiter. You may reach a point where progress plateaus until your form issues are addressed. Economy isn't something that can be perfected in a single workout. It takes time and thousands of proper strides before the form will become automatic. You should work on your form every time you run, or at least be aware of it to ensure you're not going back to old (bad) habits.
If you're a novice runner, the sooner you work on your form the better. It's much more difficult to change form that has been reinforced by years of bad habit.
Having good form doesn't just improve speed, it can help prevent injury. When you run, you land with a force three times your body weight. By reducing vertical oscillation and braking forces, you lessen the stress and impact on your body.
Stride rate
Improving stride rate is a good place to begin. If you have a low stride rate, you're probably producing more vertical oscillation. This means you're projecting energy and motion upwards instead of forward, which produces greater impact. Running should be akin to flying with your feet briefly touching the ground.
An elite runner's feet touch the ground for as little as one tenth of a second per stride. The more time your feet spend on the ground, the more energy you're wasting. You want to aspire to a stride rate of 180-190 strides per minute.
If you're a beginner, in all likelihood your stride rate is closer to 170 strides per minute, or lower. Don't worry about your stride length; your stride will naturally lengthen as your stride rate increases. Count your left or right foot strides for 20 seconds. You should be in the 30-32 stride per minute range. Increasing stride rate will initially feel awkward, and may seem like you are taking "baby steps" while running, but this is a good sign. Plan on taking several months and a lot of practice before increasing your stride rate. Be patient.
Stride rate drills
Turn overs: Turn overs train your neuromuscular system to move your legs faster than they're used to. You will use a short stride and fast stride rate. This may feel a bit awkward initially. Visualize a sandpiper running on the beach and move your legs as quickly as you can while keeping a short stride. Be sure to lengthen your stride at the end of the drill and don't stop abruptly as it will be hard on your body. You can do four to six turn overs of 50 meters after your run strides.
Walk/run progression: Start by walking with a fast turn over and proceed to your walk/run threshold. Move your feet as fast as you can while maintaining a walk. Now slowly and seamlessly progress into a slow run with a fast turn over. Your stride rate should be about the same. You'll find that your stride is smooth and that there's little vertical movement.
Metronome running: A metronome is a timing device used by musicians. It can be purchased at your local music store for around $25. Be sure to get a small, portable, battery operated unit. Dial in 180 beats per minute on your metronome and match your footfalls to the beat. Once you get your rhythm down get on a treadmill and practice maintaining 180 s.p.m. at a variety of speeds and grades. You can also download a digital metronome and save it to your MP3 or CD player. Go to http://www.milsoftware.com/crystalmetronome/ for details.
Foot strike
Your foot should strike forcefully directly under your center of gravity or hip. Visualize a line from your belly button to the ball of your foot. If your foot lands before or after this point, there are braking forces that will decelerate you.
I recommend a mid-foot strike just aft of the ball of the foot. A mid-foot strike limits the amount of time your foot spends "rolling" along the ground when compared to a heal strike. The less contact time your foot has with the ground the better. Use a quick contraction of the muscles in your lower legs during push off, or a "pawing" motion.
Foot strike drills
Barefoot Running: Running in thickly padded shoes on even surfaces does not make the muscles of the foot and lower leg work very hard. You also transfer more of your energy to your shoes and less to the ground. When you run barefoot you naturally use a forefoot strike and strengthen the foot and lower leg muscles. Not only does this give you a better foot strike feel, it helps prevent injury. Start by spending as much time as possible walking in bare feet. Add barefoot running very gradually into your training, starting with just one session per week. Make sure the surface you're running on is well tended and clean of debris, such as a golf course or athletic field.
Marching: Begin by walking slowly forward on the balls of your feet, making sure your heels don't touch the ground. Use small steps, about 12 inches in length. Then raise your right knee to hip level (so that your thigh is parallel to the ground) on each stride. Draw your heel along your inseam as you raise your leg. Your right ankle should be directly under or slightly behind your right knee, and your right foot should be 'cocked' (toes pointing upwards). This will form a "Z" formation with your foot, lower leg and thigh.
Rise on the toes of your left foot as you bring your right knee to hip level. Hold your chin and trunk upright. As you get acclimated to the leg mechanics start swinging your arms slowly in rhythm with the marching stride. Use proper arm motion (see below), and don't lean backwards.
Repeat this action with the opposite leg, raising the knee to hip level and moving through a normal walking stride for 50 meters.
Posture
When running, picture yourself as a puppet controlled by marionette with a string attached to your head. The string holds your posture vertical and perpendicular to the ground. Keep your chest out, eyes on a point about 30 feet in front of you, and head fixed. A slumped posture restricts your breathing. Keep your hips and back erect, creating an overall "tall" posture. Keep all your motion projected into the forward plane and avoid any lateral or vertical motion.
It's hard to correct your form if you can't see it. To get some visual feedback, position a mirror at various positions around your treadmill, or better yet, use a video camera equipped with slow motion to video yourself running.
Posture drill
Hips tall position: Stand with feet at a comfortable distance apart and slowly rise, supporting your body high on the balls of your feet, while squeezing your abdominals.
Arm motion
Your arm motion acts as a counterbalance to your hips. If you have a stiff upper body while running, your shoulders will rotate causing an opposing movement of the hips; again, wasted energy. Try keeping your shoulders loose and your arms swinging like pendulums from your shoulders. Your arms should work in the same rhythm as your legs. Keep your hands relaxed and thumbs up.
Maintain a fixed 90-degree angle at the elbow and make sure your arms don't drop below your waist. There should be no movement at the elbow when running. Your arms should work freely forward to back and should not cross the midline of your body; remember all energy forward. Keep your hands loose, thumbs up, and don't clench your fists.
Arm motion drills
Side Brush: Gently brush the side of your ribcage with the palms of your hands as your run. If you have a fixed angle at the elbow you can't "reach" with your hands.
Pendulum: Concentrate on relaxing your shoulders, especially the trapezius muscles, by performing a few shoulder shrugs. Now swing your arms loosely front to back, keeping a fixed 90-degree angle at the elbow. Make sure you're not rotating your shoulders. Slowly speed up the movement while maintaining a relaxed swing. Are your shoulders relaxed?
Strides
Simply put, strides are running with perfect form. I recommend you perform strides at the beginning of your workout before you're fatigued. Work on your key limiter. Start off slowly running 100 meters concentrating on your form. Walk back to your starting point and gradually increase speed and distance as you maintain perfect running form. Strides are a great warm-up activity and should be an integral part of weekly training.
As you can see, there's a lot more to running than just moving your body faster. If you're reinforcing bad form you're working against yourself. A lot of economy problems are just bad habit, but some are caused by an injury, biomechanical problem, or flexibility issue.
The best course of action is to get some professional eyes on you and identify your individual issues. I video my runners on a treadmill and play different shots of their stride back in slow motion. This gives very precise visual feedback on what they're doing right and wrong. Don't try to change everything at once, or overnight. Your pace may actually slow slightly as you adapt to new form, so be patient.
Work on the most glaring problem with your economy and perfect it, then move onto the next. I never attempt to work on more than one or two things per session. Work on flat terrain, since it's easier to focus on form. Finally, realize that even if you are an experienced runner with great form, it's still a good idea to check your economy regularly. Old habits do die hard.