First guide, enjoy!
Notice the Morello's evil tome costing a whooping -145 gold ;)

This just seems to be the best build after playing quite some games. Though I'm merely level 15, I can't imagine a game with lux without these items. At first, take a Meki Pendant for mana recovery, lux IS extremely mana hungry. Take these potions too, you can't afford to die in the beginning... You'll notice quickly you still don't have enough mana, so grab anotehr one. Logic :) As soon as you have two of em, you will be annoyed by the CD of your speels, so grab the Fiendish Codex, work towards a morello's Evil Tome to get some more Mana Regen and your first sirious portion of AP. I'm not satisfied with the CD's, so I grab the Ionian Boots of Lucidity for spell spamming, yes, you start to spam spells at the mid, cool ;O This already makes you a hard to takedown champion ( binding is so <3 ). Now it's time to upgrade your other meki pendant, work towards a Tear, this will increase your mana early, and at last the Archangel's staff. No need to describe hm :)? Next thing to buy is a nice deathcap for some serious damage. Grab a sheen for giving your passive some more Oomph and a Lich Bane when enough. Mostly the game is already over, so I rarely buy the Hourglass. If you can, BUY :D At the late game your opponents get tougher, the active of that thing is handy ;)

Don't be afraid to take a void staff instead of something else due to the huge magic resistance of late-game champions.

+ Nice Spellstreaks
+ Powerful at all times
+ Damaging slow spell
+ Nasty passive
+ Lucent Singularity will slay an entire wave ( except the boss of the wave, don't know how it's called )
+ Lazr

- Passive get's useless later on, 190 DMG on a champion with 3k hp won't hurt that much.
- Squishy
- Nasty CD's

Flash to teleport to the other side of a wall
Ghost to chase / run off easier.
Need to say more?

Clarity becomes useless in the late game, I recommend you to not pick it.

Lux is unique to me for her "flashy" style.
Throw Binding in the face, use your attack, throw a Singularity at him and make it boom instantly ( use the e, clicking might make you misclick, timing is essential! ), do not attack, but use you're Finales Funkeln ( it ignites the passive effect and refreshes it ) auto-attack if not dead yet. Using this combination at lvl 6 has a high succes ratio. Try to time so nicely that he is dead in less then 5 seconds. It's possible. I said it's possible right?

Short, Singularity, done.
Try to gank a wave from the sides to destroy it in one shot. Not Recommended in mid/early.

Too early, I want tier 3, not tier 2. I thought I had a Quintenessence of Force for more pow.
I picked the runes that would suit her the best. I can't test it though ._.

Binding and Singularity is are quite important, barrier should be upped really late. You shouldn't charge in when you're low on hp...
Finales Funkeln ASAP. No need to comment about that ;)