While new sails are always nice, in light air stretch is minimal and a properly trimmed old sail along with good race strategy should serve well. In my opinion light air sailing is the most challenging of all sailing. My mental image is that of hopping from rock to rock across a shallow stream. The "rocks" of course are the light air puffs scattered across the course. Getting from one to another is the payoff.
Sailormon6 and JeffH have already provided some good information. About all I can add is to use a light weight genoa or drifter and hold the sheet in your hand for instant feedback and to be able to "nurse" the sail through the power boat wakes. Keep the headsail still and full and mind the upper telltales for that is where the wind is. For more whys and wherefores see Bethwaite''s High Performance Sailing, page 281-287.