Lux is a champion that attracts a lot of players because of her laser. I was one of the many that came in with the mentality that I wanted to melt people's faces with such a long range, low CD skill. After playing her for a bit I learned how strong Lux is at zoning and farming champions in lane. I emphasize this build for players that are interested in farming really well and getting a lot of items while nuking and supporting a team from the back. If you are running around looking for purely laser snipes, this is not the build for you.

The runes I selected are just my thoughts. To be honest I haven't experimented thoroughly with this aspect of the build.
I feel that the early AP from the quints combined with the magic pen reds really make you hit hard with spells early game. This is important due to restricted mana regen before leveling up/ obtaining mana regen items. It is vital to have this to zone out the champions in lane or at least keep up the farm.
For the yellows I get mana regen/lvl just to be able to start spamming spells as early as possible. Her spells can easily clear minion waves and is almost necessary to be able to spam against strong laners to push them back or to zone out a champion if you manage to push to their tower.
I pick up CDR blues just because I dont feel more mana regen is necessary here and I have practically no CDR early game s in items so this is useful in my opinion. However, AP/lvl blues would also be extremely effective here.

Standard 9/0/21.

Not much to say, lots of caster goodies in Utility (move speed, faster summoners, lower CD on Flash, Exp, mana regen, longer blue buff.)

9 points in Offense for the 15% Magic Pen.

I open boots + 3 pots frequently to evade lot of champions in lane that have Skill shot's etc. I also open Doran's if I feel that I will be experiencing a lot of pressure early game. Excellent example is Vayne, she's FoTM atm and really strong early on so I grab a Doran's ring for a few more Q's to toss and health to survive her massive damage.

The double philosopher's stone is the most important aspect of this build. The items provide lux the sustainability to stay in lane for as long as you need. I see so many Lux's grab teleport to replenish mana and return to lane so they won't get dominated. With these you get gold, spam spells to farm, regen the damage from enemy harass. Best investment and you want to be getting these ASAP.

From the farm and gold the Philo's allow you to have you really want to pick up the needlessly large rod for AP and Sorc's to be able to do damage. Build the Rod into a deathcap next.

This is the point in the guide where you should really build Lux how you want. I build her more like a carry so I pick up a Void Staff and Hourglass and proceed.

Some good options instead are to pick up a glacial shroud--> frozen heart for your team. Shureliya's is an easy upgrade from philo's also and is a great support item.
Aegis is a good idea if no one else has. The core of this build is really important and from there it is up to the player to decide what the situation warrants.

ITEM TO CONSIDER
Mejai's: I hate snowball items because they generally only help if you're already winning. However, I grab this on Lux frequently after the Rod and shoes only because you can always position yourself in the back and still be effective in fights. This safe position and flash should keep you alive so it is OKAY to get this item. Also you laser can make you an assist machine if used properly so you get mad stacks yo.

I get Q at level 1 for team fights and a general disable for escape/attack. Some people grab E lvl 1 but I don't see why you would goto push your lane already. Q is also a better harass if you land at lvl 1 because you get in for an AA for passive w/o getting raped by creeps.

I max E first so I can spam farm/push lanes for when I want to Blue Pill and return losing minimal experience. This synergies well with the insane regen from the double philo. Also it is much easier to zone with E when compared to Q so I want more damage off this.

I get a pity point in W because it's saved me before from random things like Karthus ult and the one in a 100 games it saves you with its pitiful shield when you have no AP early on is just awesome. Later on when you have a death cap and more AP this ability can turn the tides in a team fight.

Level Laser whenever you can purely because it's shiny. 'nuff said.

I like Flash/ignite. I solo queue so these are my bread and butter for my lane. Flash is a great escaping tool.

Flash is mandatory imo but, feel free to swap ignite for:
Clairvoyance: Map hacks are always cool. (Make sure you switch a point into the talent in utility from the movement speed talent.)

Ghost: If you want another escape/chase skill. (Move point from less time spend dead to Improved Ghost if you get it.)

Exhaust: Sometimes if I feel I'll lane against an AD or you generally have trouble against an AD champ or you feel like hating on an AD champion. Grab this. (once again swap a point into this mastery from deadliness in offense tree if you get it.)

Telport: Counters Globals slightly. Same with people that Backdoor all day. (Swap a point into the spatial accuracy in Utility if you get)
DON'T GET:
Clarity: Philo Stones are mana regen. Bad late game.
Heal: SO bad late game. Minimal results early game because of your long range.

I skipped a bunch of summoners because I dont think they are worth mentioning. I guess Clarity/Heal belong there too but I actually see these on other Lux's.

Against a Strong Laner:
Survive you lane. Get every last hit possible, don't use you spells and push lane. Minimize as much damage as you can and if you see the enemy overextend Q+E combo while getting melee in between spells for maximizing damage. Against these types I get as many last hits as I can and I get to 6 use E to clear an existing creep wave and then E+Laser combo the next wave (try to hit enemy champ also) to clear wave to give some time to go buy.

Against a Weak laner:
grab every last hit and use your E on enemy caster creeps and if the champ walks to that zone detonate and AA him. You have to do your best to get E to hit creeps and champion because of how it helps in conjunction with your passive to last hit while harrasing. If you feel you can combo enough for a kill: go for an E to slow and then trap them with Q in the zone and toss an AA as you detonate the E and AA again+ ignite. This is the maximum damage you can apply with your combo. Im perfectly content allowing the champ to try and last hit at their tower while you zone them with your E so they cant even approach the creep wave at THEIR tower without being punished. Once again at 6 go for a kill with laser and push lane and back to base to buy philo's or E+laser combo a fresh creep wave and Base for items.

Most important thing is to realize you don't have infinite mana and make every spell count. If you can't land Q's don't waste mana and just get gold.

If you did well early game you should have hopefully boots + 2 philo stones. However, if things didnt go as well you should at least have one philo stone. With these items FARM. Your E is doing enough damage that you can get one AA on every caster creep and detonate for all that gold. You should be able to hit all melee and caster creeps with one E so while your minions make quick work of the melee ones you can just look for easy last hits when they're debuffed from E. once your minions are pushing try you best to harass the enemy champ. I like to spam my laser at this point on CD on fresh creep waves if I feel not much activity is going on around map (nobody going for dragons, important jungle buffs, etc.) Save your laser if you feel a fight is about to erupt but otherwise you can push and B often. WARD YOUR LANE AND ANY IMPORTANT OBJECTIVE. gold should not be a problem here and these MAP HACKS save you. If you can get a blue buff at this point in the game you have strong lane presence. See if you can grab it for the CDR/Mana regen. If someone else needs it more (i.e Swain/Karthus/Amumu) then let them have it.

Regardless of item choices you have really important things to keep aware of at this point in the game.

1. Teamwork: be with your team as much as possible whenever you go for objectives, by now your W can save a lot of damage for your ENTIRE team if you toss it correctly.

2. Pushing: You can shove any lane quite quickly so communicate with your team and shove lanes before big engagements so you can team fight without worrying about a tower being raped by naturally pushing creeps (or a BD champ that would have used these creeps for a quick tower while you are stalemating an enemy team.)

3. Position: BE BEHIND YOUR ENTIRE TEAM. The reasons for this are that you can still hit enemies at this range. You need to hit as many teammates with W as possible. You have zero defensive items so you die in 1/2 a second. If the enemies choose to focus you, you can run while you team wails on them while they're slowed through your E. Also teams that chase you more than likely will will get double binded by Q and eat a laser and whatever else damage your team has IF you're far enough back that you have time to pull this off. I DO NOT MEAN RUN AWAY IF YOUR TEAM STARTS A FIGHT. Think of yourself as a poking champion that darts in and out constantly. Dart in and launch spells and dart out while they're on CD. This takes a lot of personal practice to get right.

4. Pot up/Buff up: Try to get Blue Buff + Blue Elixir, coordinate with team but you need this CDR, (At least blue pot) to team fight effectively.

5. LASER; Whenever there is an opportunity to push back creeps/hit multiple champions USE YOUR LASER. SUPER SHORT CD DAMAGE POKES ARE COOL. Dont be that guy that only snipes kills with laser because the pokes from it are far more helpful to team.

This is my first guide, I want as much feedback as possible because I really believe in the power of this build and I want to share it. Any comments on things to add/change will help me help you see the power of this champion/build.