Every bike will shimmy, it's just a question of at what speed. Shimmy is the unfortunate combination of many factors to create a harmonic vibration in the bike.
There are a number of things that you can do to reduce the possibility of encountering it. First check to make sure that everything is properly adjusted, such as the headset, front wheel, stem and handlebars, and racks. Next, try to move as much of your load as possible to front panniers. If you are still having a shimmy problem, you can change your position on the bike to dampen it. Try clamping the top tube with your knees, lifting your weight off the saddle, or leaning further forward towards the handlebars. I'm assuming that the shimmy occurs at high speed, such as on a descent.