Oh the speed wobble, it is one of the most scary things that can happen on a bicycle.
While this may seem like it is outside of your control, there are three simple steps you can do to stop the wobble when it hits and prevent it from ever happening.
1. Try and pedal every opportunity you have during the descent or during riding at speed.
This is not pedaling to move the bike forward, instead to just keep the weight being evenly distributed over the key part of the bike, the center. Often riders form what I call the "death ball" which is they get scared, stop pedaling, and form like a ball over the bike. Their feet are parallel to the ground, with one in front of the other which basically weights the front of the bike on one side, and then the back of the bike on the other. In the "death ball" the bike will now shimmy as it is unbalanced, going back and forth responding to where the riders weight is.
2. Add more of your weight over the front wheel.
The front wheel is where all of your control is located on the bike. Here you have control of the tracking, braking, and turning. Often times when the weight is too far on the back of the bike, the front wheel will shimmy as it is unweighted and vulnerable to side to side motion from the wind or hitting an object. By simply placing more of your weight over the front wheel you can stop the shimmy as well as prevent it from happening. If you feel your front end start to shake or wobble, react quickly by moving your chest over and low close to the top tube with your head low and closer to the front wheel.
3. Use the front brake lightly and pedal into it.
Think of your front brake like a reset button. By using your front brake you can reset the bike's stability by applying it lightly and pedaling into it. This third step really brings the first two steps together, and by adding the front brake this is the fastest way to reset the bike quickly and get it out of the shimmy.