Recently bought a Shimano Acera SL-M310 Rapid Fire Shifter for under $17. It is originally designed to be Mounted cross ways on a Straight Mountain Bike or BMX Handle Bar. I mounted it on my left Aero Bar, shoving it up as far as possible without interfering with the Grip. The Clamp is real Solid and the Shifter works fantastic. It works better then the expensive Brake Lever Shifters that cost hundreds of Dollars because you can leave your hands on the Aero Bars. It works just as well as the expensive Toggle Shifters that fit into the ends of your Aero Bars and is better because you can use it with Aero Bars with a Top Loop. To up shift you can actually leave your hand on the Left Aero Bar and reach under with your Pinkie and jab the Shifter and it works precisely every time. To Downshift you can either reach up with your left thumb or reach over with your right hand and jab the downshift lever. Works Great. Very Smooth. Very Easy. Very Precise. For $17.

As per "texaspandj's" suggestion:

The first fifty years are for learning...the next fifty are for living. And I was Born 15.