If you can, pedal. If you are not pedaling, then press your knees against the top tube. That allows your body and the bike to move as one entity, as opposed to the bike wobbling beneath you.
If you can, pedal. If you are not pedaling, then press your knees against the top tube. That allows your body and the bike to move as one entity, as opposed to the bike wobbling beneath you.
What trigirl said. I'd also have the bike shop check your bike over to make sure the headset or something else isn't loose. Unfortunately, some bikes have more shimmy on the descents because of the geometry (it's complicated) than others.
If you're not confident about bike maintenance you can definitely ask a local shop to check it over. But, many things you could catch on your own by trying to jiggle components while standing still. For example, your headset and wheels shouldn't be able to move in any direction than their respective rotational axes.