Here's another excellent excerpt reprinted with permission from Human Kinetics of High-Performance Cycling by Asker Jeukendrup

"Throughout this chapter, we have used a set of reference values for aerodynamic drag area. Although these values represent good approximations to the drag area of a 70-kilogram (154-pound) rider in each position, those values are not fixed. Rather, a cyclist can influence his drag area in several ways. Riding with knees close to the centerline of the bicycle frame can reduce drag area by approximately 8 percent compared with riding with knees wide apart. This knee position will affect drag similarly whether the rider uses conventional racing handlebars or aerobars.

For riding with standard handlebars, arm position, including elbow bend and forearm alignment, can even more dramatically influence drag area. Bending the elbows allows the rider to lower his torso and thus reduce frontal area. Indeed, carefully positioned arms with the forearms horizontal and parallel to the bicycle can reduce drag area by up to 12 percent compared with widely positioned arms or straightened elbows. A wide-elbow position may result from poor technique, but it also may be due to poor bike fit and thus may not be within the control of the rider. Specifically, if the saddle-to-handlebar distance is too short, the rider may be forced to widen the arms so that they do not contact the legs. Consequently, drag area may be substantially increased because of a poorly fitted frame-stem-handlebar combination.

Optimal Time-Trial Position
Riders often ask, "How do I optimize my time-trial position?" The simple answer is, "Go to a wind tunnel and have your aerodynamic drag measured in various positions." But, of course, not every cyclist has the opportunity to take part in wind-tunnel optimization.