How do you get faster in the 100m/200m dash?

Just running isn't enough. My coach taught me something called "repeats", and they work. What you do is sprint for 300m (with all you've got), then walk for 100. Walk slowly so you can catch your breath - this isn't supposed to exercise your lungs, it's supposed to exercise your leg muscles. Then give it your all for another 300m. Make sure you catch your breath between these, but give it your all when you're sprinting. Try to beat your previous time each time, and do this about 6 times. By trying to beat your time each time, you're pushing your muscles beyond what they're comfortable with and what they remeber from the previous time, therefore making you faster. Repeats are pretty much the only thing that actually make you faster.