Notes on my build

I use this build to carry and support my team. This build has yet to fail me.
With my masteries and items, you will have your CD capped to be an ultimate supporter, and also have enough damage to scare off enemies. A good start gives better results than a bad start. Also, you must keep in mind that lux does better in the end. For ex: i was 3/7/13, and in the end I ended up getting 20 stacks with Mejai's, my score resulting in 18/7/18. This build is for more experienced LoL players.

Why flash and Ghost?

Flash and ghost are the most efficient way to move during battle. It can save you from ganks, and it can also help you get a number of kills.

Starting off

I would start off with a Mana potion, Health potion, and a Sapphire crystal. Health runes will help you in the beginning, with the potions. Farm as much as you can, until you get glacial shroud. Do not buy shoes until you have bought the shroud. TIP: Do not be offensive unless you are going for a kill. It is important to play more defensive. Do not harass either, conserve your mana. Sorcerers shoes can help you do more damage. Then finally, get all the necessary ingredients to make an archangel's staff (tear's of goddess and blasting wand). At this time you can start being more offensive with ganking and attacking, and not worry so much about your mana. Deathfire grasp is also important, and you should take that in after the staff. Usually the game only goes this far, but if it is a long game, it is important to get void staff and rabadon's death cap. These two items are your source of damage. Your CD should be capped at 40%, and now your mana should not worry you at all.

Early Laning Tips

As stated above, in the beginning, a defensive stance is necessary. (read above in red for more info) Illumination, which is lux's passive skill, is important for getting the max amount of damage output on an enemy. Try to attack after every spell when laning, because it always does an efficient amount of damage. It is important to keep Lux alive, so do not tower dive, or be foolish because every gold and exp counts!

Big fight tips

Usually you only have to worry about this in Summoner's rift. In case a big party starts up in mid lane or something,It is Very important that you are behind your allies. Even though Lux can deal a large amount of damage, She can also take a lot of damage as well. Although it is not much, always use prismatic barrier right when it comes out of CD. Constantly toss lucent singularity and snares, and try to hit as many enemy champions as you possibly can with Finales Funkeln.

Mejai's SoulStealer

I do not recommend getting this if you are inexperienced with lux. This can be a big waste of money, or it can be the best item you can own. I usually get this when i feel like taking the gamble, or when im fed.

Important Skills with Lux

Light Binding:This skill is VERY important because not only does it do a decent amount of damage, but it also snares the enemy for other hits.
Prismatic Barrier:Everyone thinks this skills is useless. It is true that it barely nullifies any damage, but there is one important use for this. It can saved you from ignite, malefic visions, and other DoT (damage over time) spells. It feels bad just to wait for your teammate to die with one bar of health getting ignited right? This is a valuable skill that can be great for this reason.
Lucent Singularity
There isn't much to say about this one. It does the same damage as the snare, and it barely slows. It is necessary for a combo however. This can also be used as a temporary ward in bushes.
Finales Funkeln:This is by far obviously the best skill. The best part is that with the items and masteries I have at lvl 18, The CD is 24 seconds! This skill is great to start of big fights, and a great way to literally "snipe" a lucky runaway.
The CD on this skill should be taken advantage of, and should be used whenever at least 2 enemies or a low hp enemy is in sight. If you miss, no big deal. You only need to wait 24 seconds right?

Combos
Lux can kill very quickly if used right. It is important to mash the keys to use spells quickly one after another. This is my main combo which i use on full hp enemies.
Light Binding>Lucent Singularity>Deathfire Grasp>Detonate>Finales Funkeln

If one can aim greatly, this way can take time to kill the enemy and might require ghost and flash for the kill. However, this combo takes most of the enemies hp away.
Deathfire Grasp>Lucent Singularity>Detonate>Basic Attack illumination>Light binding>Basic Attack Illumination>Finales Funkeln (should be dead by this, if not, illuminate them)
The hardest part of the combo above is getting in the attack illumination followed by finales funkeln after the light binding snare, because at this point the enemy will sense danger and flee. If you have trouble doing this quickly, this combo does almost the same amount of damage.
Deathfire Grasp>Lucent Singularity>Detonate>Basic Attack illumination>Light binding>Finales Funkeln (should be dead by this, if not, illuminate them)
The simplest combo I know is this:
Light Binding>Lucent Singularity>Finales Funkeln>Detonate
I do this if deathfire grasp is still on cd, or if I know this will be enough to kill the enemy.