I think that Sailormon6 has hit on many of the key points of light air performance. A few minor points;
First of all, get out there and practice. There are few things that are harder to get good at than light air sailing.
Sailormon6 is right about heeling your boat in light air but you want to heel your boat over for a number of reasons besides maintaining sail shape. In light air, what ever air movement exists will often have a large vertical component to its direction. Heeling the boat over exposes more sail area to that large vertical component. In light air it is important to reduce wetted surface. Waterline has no bearing on speed in light air. Wetted surface is everything drag wise. You want to trim (fore and aft as well as amidships) so that the boat is floating on its most cylindrical rather than eliptical sections of the hull. This usually means heeling the boat over to leeward and trimming down in the bow. This normally means getting your crew''s weight clustered up near the leeward shrouds and in flat water even forward of the shrouds. Avoid moving the helm as much as posible as a turned rudder offers a lot more resistance than one that is close to neutral. That said, heeling you boat over will give the boat a little weather helm and some helmsman prefer having a bit of ''feel'' in the light stuff.
Even small gains in speed in light air are important as they produce more apparent wind and allow you to sail from puff to puff and ideal avoid the big holes. In the really light stuff tacking or jibing can really kill you as it can take a huge amount of time to regain your speed and lost apparent wind. You often need to power up after the tacks and then reshape the sails as the boat achieves more speed.
With all due respect to sailormon6 who clearly is an experienced and knowledgeable sailor, I strongly disagree with his recommendation regarding a shelf foot and flattening reef being good for a light air mainsail. Shelf feet and flattening reefs are really outdated technology. In modeling airflow at slow speeds along the foot of the sail, it became apparent that instead of acting as an end plate and directing the air smoothly aft, (as was the original justification for a shelf foot) the vertical component of air moving along the sail, resulted in the shelf producing large amounts of turbulance which meant lots of drag without gain in drive.
The current theory on mainsails for light air is to use a loose footed mainsail. With a properly cut loose footed mainsail you don''t need a flattening reef to blade out the sail as the wind speeds increase.