Same way you would with an ordinary brush. ;)  Click the arrow pointing down on the top of the toolbar next to the brush symbol (this is assuming you have CS, I don;t know if it's the same with the other versions).  This brings down a drop-down list.  Select the one that says 'Airbrush 50% flow'.  Then change the 50% in the flow tab to 100%...and....tada! ;)