I recently was asked the following question: "Why do heavier riders descend faster than me?" This is a very good question that many of us have considered at some point during our rides. The answer to this question actually is quite fascinating.
First some math. The following can be found on NASA's website:
When descending, riders encounter two "forces" (physicists, I'm using some terms generically): aerodynamic drag; gravitational force. In a vacuum, objects of different masses will fall at the same speed, as the resistive force of aerodynamic drag is not present. Fortunately (or unfortunately, if you happen to be a very light rider), we don't tend to ride in vacuums--we have to deal with aerodynamic drag. This is where things get interesting.
In the most simplest of terms, terminal velocity is the point at which acceleration for a given object cannot increase--the object's speed, in other words, will not go above a specific value. In an atmosphere, objects of different masses, but of similar surface areas and profiles, will achieve different terminal velocities. Heavier riders can descend faster than light riders because heavier riders don't have significantly more volume/surface area than their lighter counterparts, despite possibly major differences in weight. Even though I weigh 50lbs more than Elaine, our respective surface areas aren't that much different (though Elaine might beg to disagree), so my terminal velocity (this is a scarey phrase, given my history!) will be higher than hers--I will descend faster. All bets are off if we decide to descent in a vacuum, however.
What lighter riders can do on descents is work on their position on the bike, tucking in as aero of a position as possible. Additionally, ensuring that lines taken in turns are efficient so that speed isn't scrubbed by unnecessary braking will help. In terms of equipment, a set of deeper, aerodynamic wheels might help close some of the difference--say, 60mm rims. Keep in mind that with today's tech, aero always will trump lightness (most aero wheels are very light as compared to wheels ten years ago), so try to use wheels as aero as possible.
See also : http://www.cyclingnews.com/news/tech-hed-launches-descending-special ;) Note the date!