Lux is a range ap character. She excels at fighting battles from the back of team fights while supporting her team. Her ulti is great for doing damage either in the beginning of fights or at the end of the fights. Her Q ability snares 2 targets, the target in the back gets half the damage and snare time. Her W ability is a boomerang skill shot that creates a temporary shield for her and her allies that get hit my the ability. Her E ability shoots a ball of concentrated light at an area. You could press e again to detonate it or let be there and it will detonate in 5 seconds. Her passive is illumination which lights up a target. An auto attack would light up the target and do more damage.

Pros:
Her ulti is great for kill enemies that are running away, just make sure its lined up correctly. She could harass champions very easily with her skills. Her ranged skills makes it very easy to fight in back of fights. Ulti on short cooldown.
Cons:
Very Very squishy early game.
Very long cooldowns for her basic skills
IM FIRING MAH LAZZORS

I take Magic penetration marks for early game harassment, mana regeneration per second seals so i could spam my abilities without worrying about running out of mana, ability power per level glyphs for more ap :), and flat ability power quints for more damage overall.

I take the standard 9-0-21 masteries cooldown reduction and mana regen per second.

I start out with a dorans ring which gives me over 30 ability power early game for really high damage in lane. Depending if your doing good in lane or not you should either rush a mejai's soulstealer or just buy magic penetration boots. But i required every lux to get a lich blade so that when you do a combo, your next spell will do a lot more damage than the previous one. After getting a lich blade, i either get a deathcap, void staff, or a hourglass. But if you are doing really got, get another deathcap and a hourglass.

Once you get a lvl 2 you could start using your combos. Start by casting a light binding snaring the target do one auto attack and cast your E in FRONT of them, then do another auto attack. keep harassing all the time. When you get to lvl 6 this is when you fun starts. look for an enemy that is low in either top or bot and run tower them. when you do see them, hide in the brush next to their turret and see if you can cause light binding. once you hit light binding, immediately use you ulti. if your lanning and you see someones low, cast your E in front of them then ulti immediately, right after your ulti, if theri not dead, then press you E for a kill.

I take clarity in case i run out of many early game and flash to get away for enemys when either my q is on cooldown or im panicking :P

Cast your E in caster creeps in the back and auto attack for more damage, this could be a very efficient way to farm minions if you time your auto attacks correctly.

All i could say is IM FIRING MAH LAZZORS :D