Hey all,
My name LoL user name is Summoner Popeye, and I'm currently a Silver 1 player. I've played Lux since season 1(she was the 2nd champion I ever owned), and feel I have a pretty good understanding of her strengths and weaknesses. In roughly 250 ranked games with Lux this season I currently have a 7/5/10 average and 60% win rate. While those numbers are not overly impressive I realize, at least 1/3 of those games were played as support. Anyways enough rambling. On to the build!

All Runes Tier 3
Marks: 9 x Magic Pen
Seals: 9 x HP/lvl
Glyphys: AD Opponent: 9 x Flat AP
AP Opponent: 6 x MR/lvl, 3 Flat AP
Quints: AD Opponent: 2 x Flat armor and 1 x Flat AP
AP Opponent: 3 Flat AP
Lux v AD: 7.8 magic pen, 216 HP/lvl, 8.5 Armor, 16 Flat AP
Lux v AP: 7.8 magic pen, 216 HP/lvl, 18 MR at lvl 18(1 MR/lvl), 18 Flat AP

Lux is pretty squishy starting out which is rather inconvenient since her passive is reliant upon AA's. The HP/lvl give her less of a "Yes, your right Leblance/Fizz/Zed/etc, you can delete me if I make a small mistake with snare on cd" look, which generally speaking I think is a good thing. The Armor or MR helps with that as well.
Don't get me wrong I DO NOT intend to GET HIT VERY OFTEN AFTER LVL 6, but its always nice to have a little extra time to react to a gank or all-in.
If you like to play AP casters I suggest using 2 rune pages. One for each opponent type.

21 OFFENSE-Get core cd,ap/lvl,AP,5%AP per AA,pen. The fillers are a personal preference. I like to have feast though and will explain why in farming section
9 DEFENCE( Health Regen, Block, HP, Juggernaut)

Flash- Duh! no escapes
Tele- I prefer tele:
Over Ignite- As it nears mid game you are going to want to be in ignite's cast range less and less. Ignites primary functions are to reduce healing or lifesteal via dps or secure kills. I'm not letting a heavy regen or lifesteal chara anywhere in range to AA me so they arent lifestealing and i have no need for the dps as they could just heal or lifesteal it back after its over. Lux has a fk'in LASER to secure kills.
Over Heal- I do not dislike heal on lux, but i just find greater value in teleport since it allows a full heal, mana refill, and purchase w/o giving up free tower damage. Not to mention it can be game changing for swinging lane fights.

Bottom line: Tele/Flash is the tits, and you shouldn't leave home without it!

Normally:E,Q,E*,W*,E,R.....
You get the drift. Max E 1st, then Q, then W last. Depending on the game I may give W a 2nd lvl before I would be forced to, but prefer to go hard E,Q,W after lvl 6.

*On occasion if myself or the opponent is low and tower hugging I will take W at lvl 3. Allows a little bit of tower dive room or survive room if they dive you.

Lvl's 1-4: Ok so your lvl 1 and only an E to your name so use it wisely. Basically for these lvl's its all about maintaining balance between E'ing, for farming and E'ing for poke, until opportunities present themselves. E'ing minions w/o hitting opponent, and they will most likely begin to press you right after you use it knowing that your most reliable source of damage is now on cd for some time.

General farming concept- E has a huge hitbox so ideally when wave first arrives identify fighter minion most likely to die first, wait till is killable with E explosion only, cast where detonation will mark all minions with Illumination passive and detonate quickly for kill on 1st fighter, locate next lowest fighter who should be at least half health, wait till Health = AA + detonation of the illumination mark to start AA and kill 2nd fighter. Repeat this process each wave for lvl's 1-4.

Mark may have disappeared at this point which now leaves you with the perdicament of waiting to try and just aa last hit and save mana, or cast again and repeat process with 3rd fighter as detonation kill and casters as follow ups. Base this choice on how fast your opponent is pushing lane. You do not want to over extend yourself. Also consider whether or not you can hit him, 3rd fighter, and caster with the same E or not. If he will allow you to do this without having to trade then it is worth, the risk of extending and the mana it costs. If he is not positioned where you could accomplish this then just sit back, AA last hit, and watch for his poke attempts for the rest of wave.

At lvl 5 my farming strategy changes. You should now have 3 lvl's on E giving it sufficient damage to quickly dispatch caster minions when combined with the detonation of passive via AA. Cast and detonate as soon as minion fighting formation takes shape and detonate to mark all 6/7. If your opponent is retreating when you move up simply walk up and AA each caster one after the other for 3 cs. Most of the time there will be the opportunity for a 4th AA illumination consumption kill on a fighter minion as well. Find it, kill it. Last hit others or consider using E again if there is a large minion in this wave or you can poke with it as well.

This trade-off is constant. Casting E is increasingly more worth the mana cost if you can mark wave for later Illumination detonation(easy last hitting), detonate for kills, and poke all in the same E.

Farming from this point out is a breeze with e/illum detonation.

NEVER USE Q TO FARM!!!!!!! Rare exceptions after aces maybe would be OK, but if there are enemies alive, in play, and not on mini map do not do use it ever. Lux has 0 escapes so her CC is very important to her survival.

Lux has alot of over looked utility value in the fact that all of her abilities are skill shots and are unaffected by terrain. This adds alot to her chasing abilities in or around the jungle as the paths are windy and as snare can take a direct path.This will essentially allow you to close gaps that would normally be just too far for a snare to catch.

Everything lux does has the potential to affect more than 1 target. When you consider she has the ability to E and Ult damage(which would also consume and remark illumination) the entire enemy team and shield your entire team it becomes clear she is a valuable team fight champ as well as a high utility one.

Always be aware of the opportunity to snipe fleeing enemies with a well placed ult(establish past, and lead a bit), or snipe objectives. I cant tell you how many times I have sniped a enemy blue, dragon, or baron just because the jungler either did not have smite or know how to use it properly. Relatively short cd on it so if I think I have above a 33% chance of stealing objective or landing a kill snipe I will let her rip.

I left a Item build off as I do not really have a set one I have been working with at the moment. With the changes in patch 4.13 I have been toying with a tankier lux whose core is R.O.A, Lichbane, and Cap. Finished with morrello, void staff, or liandrys(1 of the 3 must be left out)

Typical mid core is still grail in my book even with the nerf, and in most cases would be substitued for R.O.A in build above. If no R.O.A then no lichbane, as Lux lacks the escapes or durability to truly capitalize on it IMO.