If you are reading this, you probably are interested in playing Lux . I am neither high elo nor have I won 500+ games with Lux. I barely reached level 30 a month ago (maybe less). The guide I found most useful for Lux was shm0's **TOP SECRET** #1 DAMAGE Guide for Lux - Melting Faces 101. This guide will teach about everything I put in here, and probably better. I prefer slightly different item order however (if you notice, same items)and different masteries. Now please do not go hating on me saying "You stole shm0's build and are trying to look good for it". I feel my way of going about playing her is just different enough to put out here.
Like his build, my goal is to get a ton of ap, with just a bit of MP and CDR on the side.

Lux, in my opinion, is the best ranged mage in the game. She does need some slight buffs at the moment, but her kit is awesome.

Pros:
Damage Snare (Passive Mark Added)
AOE Damage (Passive Mark Added)
Snipe damages all enemies the same! (Eats Passive Mark)
Wide Range (Caitlyn just barely outranges her I believe)
Hilarious Jokes

Cons:
High Mana Costs
Shield is barely useful (Needs a buff)
Needs some buffs to damage and mana costs
Can't defeat lifesteal very easily

I prefer to take Flash and Clarity . As of now, most would say flash is the best escape and initiating spell. You can get away with low health or catch up to a fleeing enemy. Clarity is a crutch spell, right? So why do I take it? Clarity sustains you with the mana you need when taking mid lane. It also can change a team fight if your team is low on mana. It also gives the element of suprise if your foe sees you on low mana, engages, and then you clarity and blow them away. (: Fun right?

I tend to prefer the 9/0/21 over the 21/0/9 masteries. Why you ask? The offensive masteries are now alot more viable for ap characters, but I prefer the 6% cooldown reduction, 15% spell reduction, 216 mana help, and 5% experience gain to the 5% extra ap and 18 extra ap.

Morello's and Mejai's are both great picks. The deciding difference is the game you are in. Builds are never completely set in concrete. If you happen to be destroying, Mejai's is the way to go. The extra chunk of Ap can be extremely useful, and fully charged has the about the same cooldown as Morello's (may be more, may be =, may be less). If you are not doing so hot or are going about neutral, go the safe route and take Morellos. The mana and immediate CDR can be very useful.

My items are pretty standard for a Lux build. Sorcerer Shoes , Morello's Evil Tome , Rabadon's Deathcap , Lich Bane , Void Staff , Zhonya's Hourglass .
The first four items are pretty much non-optional (except Morello's). Void Staff and Zhonya's Hourglass, however, can be very optional. If the other team is stacking MR, the Void Staff is the way to go. If not, perhaps invest in a Zhonya's Hourglass, Banshee's Veil , or another Deathcap (unfortunately passives don't stack ): ) If they are full of 2 shot mages, a Banshee's Veil would definintely be very helpful. Play against what the enemy is playing. Zhonya's can be soooo helpful because of it's 3 second invulnerability.

I do not go for ROA because of one main reason. The way I play Lux is AP Glass Cannon. It may or may not worked in ranked. Hopwever, this guide is veered to the everyday person that wants to play Lux and be successful, not the level 30's playing ranked trying to achieve the best ELO. Yes the mana is helpful, but Lux having only 400 extra health is not going to make a anthill of a difference come lategame when every battle is you either kill everyone or get focused and are killed. However, Zhonya's could be more useful for escaping a battle alive and it gives you more AP.

Like I will mention later, Lux is all about burst and harassment. I will now take te time (upon request) to explain her skills.
Lucen Singularity -This is a skill that makes Lux a beast. Not only does it provide a great harassment tool, but it also demolished creep waves around level 17. Even better, it can be used as an escape tool! If an enemy is chasing you and you have low health throw your bind and just a second or two later your E, then your shield, you should be fine. If not, Flash away and repeat skills.
Light Binding - This is also a very versatile tool. Use ths to snare an enemy so you can set up a kill for your team. Save a teamate being chased and get endless thanks from them. Bind an ult to demolish a fleeing enemy. Save your sorry hide like just explained in E.
Prismatic Barrier - This skill is very underrated, even by myself (thanks for the perspective Toshabi). This has saved me and teamates quite often and I did'nt even realize it. This is the only shield that I know of that can properly thrown, shield your entire team not only once, but twice!!! :)
Finales Funkeln Finales Funkeln This skill is in my opinion, just about the best ultimate in the game. You can use it to snipe fleeing enemies, destroy creep waves at base (if you are losing), and even steal Baron. There is not any more thrilling feeling than sniping an enemy that has been bragging and smarting off to everyone the whole game and thinks he had escaped danger. :)
As for skill order, once you have harassd them down to 1/2 health or so, bind with Q, hit with E and blow it up immediately. If they are still binded, go ahead and ulti right away. If not, sit back, set your scope, lead them and snipe.

I always start with tier 1 boots and 3 health pots. Unless another champ really wants mid, you should always go mid as Lux. The pots and Clarity provide great sustain and should hold you until a gank or you kill the enemy. Many go with Doran's ring, but I prefer to only get items that build into something later on. You should take you E Lucent Singularity 1st as this is your best harassing tool. Once you level up, get a point in Light Binding. Q,E is your best combo before you get your ult. Only use Q when you need too, and keep harassing with E. Do not overuse it, but any time the enemy is in range and you are sure you can proc your passive, throw it out, burst it, and get out a.s.a.p. This skill eats up mana like no tomorrow, but that is why we have Clarity. By level 6 (best conditions) you should be able to snare, E, and ult for the kill. If not, they will head back to base and so should you. If you have enough money, purchase your Sorcerer's boots. If not, buy a meki pendant or amplifying tome. Supposing you get a couple kills, by your 3rd trip or so back, you should have at least your boots and a fiendish codex (hopefully a blasting wand too.

At this point you should have Morello's and be starting on your deathcap and be in the general vicinity of 100 ap. Throw in an occasional gank or two if you are able, but if you ave killed the other mid person a couple times, you should be dominating the mid because of your items and higher level. Be careful for enemy ganks, if you get too cocky or overextend, they will crush you in the blink of an eye. The enemy will not be happy if you start getting "Dominating" or "Killing Sprees", so take care.

Come endgame to the mid-late middle of the game, team fights should start happening and becoming a regular thing. Stay back a little, but be in close enough range to throw your E in whenever you can. If you can, bind their carry, or anyone not in the middle of the group so your team can focus them. You can either use your ult to get a kill or two at end of teamfight, or to take all the enemy down to 1/3 health or so midfight. Lordwilling, if you have a competent team, ya'll should slowly eliminate their turrets and push your way into the base for victory.

These are a list of people that could make Lux's life extremely difficult and why

Warwick - You waste all your mana on throwing E's and his health won't stay down!!!

Tryn - His ult sorta counters yours and also has good sustain and chasing abilities. If he uses it at the wrong time though, he's a goner.

Morgana - Her snare+ pool can make Lux's day go from good to aweful.

Sion - Not only Lifesteal, but a snare and shield to boot!

Caitlyn - More Range than Lux. Piltover Peacemaker + Ace in the Hole= Instadeath

Well, I'm not sure what ellse to really say. Any advice and constructive criticism is greatly appreciated. I hope to help anyone who wishes to play Lux to her full ability. I will update it whenever I get the time. This is my 1st guide, so please tell me whatever I did wrong.