To add to nanoboy's excellent tips, I'd say that when placing your light infantry on the flanks of your line, it can be good to back them up with a unit of fast-ish melee capable cavalry (light dragoons, dragoons, militia cav or Medium cavalry -not cuirassiers or hussars).

I like to use light troops to provoke an engagement, take out isolated enemy artillery crews (especially if they have the "tread carefully" ability), and as a last resort: sacrificial stalling of enemy flank attacks.