Most riders today will install their aerobar shifters pointing downward, as in the image below of one of Lance Armstrong's 2009 Tour de France TT bikes:
In this configuration, to move to larger rear cogs (re: easier gears), the rider has to reach down and pull up on the right shift lever; to move to smaller rear cogs (re: harder gears), the rider has to move his hand in front of the right lever and push down.
In my experience, this is not a natural ergonomic motion, as it requires a good amount of hand movement to actuate a shift. The awkwardness of this motion can result in a greater chance of shifting errors, even with indexing levers.
I've always installed my aerobar shifters pointing upward (i.e., reverse of most of today's set-ups) to minimize hand motion and to increase comfort while shifting:
In this "inverted" configuration, pushing forward on the right lever makes it easy to precisely shift into larger rear cogs, even when running shifting in friction mode. To move to smaller cogs/higher gears, the right lever is simply pulled backwards. Over the years, I have found this to be a much more natural and comfortable motion; a number of small efficiencies like this collected together can result in a disproportionately positive increase in performance.
Of course, the new generation of electronic shifters have the potential of being the most ergonomic solution of all.