The 400 meter dash marks the boundary between the sprints and the middle-distance running events. You need to be fast in order to win, but you also need enough endurance to get you to the finish line, which is easier said than done. Strategy is also an important part of the 400 meter, since the race lasts considerably longer and has more turns than the other sprints.
Start by exploding out of the blocks and accelerating to a comfortable race pace. You don't want to accelerate as hard as you can or you'll run out of energy, so back off the power a little and focus on finding the pace that's right for you.
Take the first turn as you would during a 200 meter dash. Turn your shoulder down the track as you round the turn and be careful not to let the centrifugal force of running the curve throw you off balance.
Relax as you round the first turn and float down the back straight. Focus on maintaining the pace you've already created.
Aim to hit the 200 meter mark about one to two seconds behind your fastest 200 meter time. This ensures that you have enough gas in the tank to keep your speed up for the rest of the race.
Focus on maintaining your form and speed through the final turn. You'll probably be extremely tired by this point, so you'll consciously need to put out more energy in order to stick with your original pace.
Stay relaxed through the final 100 meters and push for the finish. The key here is to keep your running form in good shape, even though the fatigue in your lungs and your legs will be trying to break it down.