NOTE: HELP ME IMPROVE THIS GUIDE PLEASE BY PLACING COMMENTS!

This is a build for Lux I always use myself. I played some very good games doing this, like 9-0, 14-2, etc. Just leave comments and I will try to make it even better.

This was my first 3v3 with Lux. I took Clairvoyance instead of Clarity, because my allies had Clarity and I absolutely loved it.
This is one of the games I played. It was my first ranked match, cause I just got level 30. This was a 4v5, cause our mundo was afk and joined at the last minute of the game. I don't have that much kills, but i do have lots of assists.

Well, I think the runes are pretty obvious: Magic penetration Marks for doing more ability damage, mana regen Seals cause in the beginning of the game this really is a pain in the *** and cooldown reduction Glyphs and Quints for the ultimate.

I take 21 points in utility, for the cooldown reduction. It is very important to do this in the advantage off her ultimate.

9 points go to offense, because it is just enough to get the magic penetration en you can even get more cooldown by taking it.

I start with Lucent Singularity, because it is a very powerful spell, great for killing minions, escaping and doing damage. I max this after taking the shield once.

Second, I take Light Binding. It is great for escaping and even more useful to snare people for your ultimate. I max this second.

Prismatic Barrier is pretty good in team fights and for escaping, but I don't max it before the other once are maxed. I saved my teammates more then one time from Karthus ulti.

Finales Funkeln, her ultimate, is a very good finishing move. Most of the time, other champions run away in a straight line. This is verry usefull for Lux. Her ulti does massive damage and will kill every enemy at low health immediately. I recommend to use it as a finishing move and not for just doing damage.

I start with Meki Pendant +1 Health Potion and 2 Mana Potions to stay as long as possible in a lane. An undefended turret can make you lose the game. When my potions are used and I really have to go back, i upgrade Meki Pendant to Tear of the Godess, so i can allready build a bit mana and stack the passive. Next go the Ionian Boots of Lucidity, cause I absolutely love them. Most of the time, I also take Mejai's Soulstealer, cause usually I can get many stacks. Then, what I see like a MUST for Lux and almost all other mages: Rylai's Crystal Scepter. It gives a good amount of health, and good ability power. And for chasing/escaping, I still have the slow. Then, finally for some of you, i take Rabadon's Deathcap. The reason I take this item that late in the game is because I first want health, mana and mana regen first, but you may prefer to take it a lot earlier. My last item will be Deathfire Grasp, to deal more then 50% of the enemy's health in a fight with the active, when you are fed.

Why not Lich Bane?
I prefer not to take Lich Bane cause all her spells, yes ALL her spells are multitarget. Lich Bane will just cost more money and won't give that much more advantage at farming and fighting, but I will explain that last one later. Also, her spells are all long ranged, much longer then an auto attack. If you got to come forward every time you cast an ability, you can get seriously damaged.

As summoner spells I go for Clairvoyance for assisting your team, make yourself almost ungankable when placing wards as well and of course placing your ultimate, and Flash for escaping/chasing.
Other choices:
- Teleport, for defending turrets and staying with your team
- Ghost, some people prefer it
- Fortify, very usefull when playing against aggressive champions/players
- Clarity to stay in your lane long enough.
- Cleanse, for escaping enemies, however it is unlikely they will focus you on the back line

Not recommendable
- Ignite, cause it needs range and you won't fight on the front line (I hope)
- Exhaust, same reason and you can slow and snare

Don't ever pick]
- Heal, because I hate the spell. I hate seeing people use it and that's why I don't use it myself.
- Revive, a good player shouldn't die...
- Rally, ARE YOU CRAZY?
- Smite, cause you are no jungler...

Farming with Lux is extremely easy. Just use Lucent Singularity on that annoying bunch of minions, and take them off one by one with your passive: Illumination. Also, when mid-laning, I recommend annoying your enemy with firing the bolt to where he is. He will have to go back a lot then.
Also, when you see a huge bunch of minions taking down a friendly turret, don't hesitate to use your ultimate, cause it has a very short cooldown!

Game Start

Playing with Lux can be tricky, especially when you have ranged enemies. Still, to my personal opinion, she is one of the best in the game.
When starting the game, claim the mid lane (or up in 3v3). Farm as many minions as you can, but your mana pool is not unlimited: you have only 2 Mana Potions and a Meki Pendant. Try last hitting minions with Lucent Singularity and last hit the others while they are Illuminated. This way, you should easily get money.
Try to annoy your enemy as much as possible with Lucent Singularity, Light Binding and Illumination. Maybe you will get allready some kills then.
If you have been farming well, you might allready have your Mejay's Soulstealer at level 6. From then on, the REAL annoyance can begin. Do just like you have done before: bully the enemy. But this time, they won't have an happy end. As soon as they walk out of your Line of Sight, use Finales Funkeln (and if you have taken it, Clairvoyance) to get him when recalling.

Mid- and Late-game

When you have killed some minions, and let's hope some champions too, you should have Rylai's Crystal Scepter pretty fast. Now, you can really start slowing the enemies and assisting your teams. But ALWAYS stay behind the fighters and tanks. Come forward to cast spells and sometimes use your passive. This is the primary reason why I don't choose Lich Bane. Don't hesitate to use your ultimate when the enemies are running away, even if they aren't at low health. Because you have Rylai's Crystal Scepter you should use your ultimate to slow all those running chickens.