Salutations,
This is my first Mobafire build and I'm glad it's on my main. Lux is a very versatile champion in a sense that she can either go full support or full AP carry, or a combination of the two (full support lacks some damage, full AP lacks some support). Some of you may be thinking that if Lux goes all AP then she can support via skills?! Wrong. Well, not entirely, yes Lux's shield and snare and slow are all good supporting abilities, however when I play support lux (due to another AP carry on the team, e.g. Anivia, Malzahar, Brand, etc) I tend to buy more aura (not to be confused with mana-whoring) and survivability items. I will add a support section later, but in the mean time, have fun winning games with this build. XD

For Marks I chose the general magic pen that many casters need.

For Seals, I chose Armor instead of MP5 because you have clarity, and while it's on CD, you should try to hog the Blue buff since I don't build CDR in the build and it provides infinite mana regen.

For Glyphs, you have a choice here, you can either get the flat AP or you can get CDR. I personally have a runepage for each when I play Lux. I tend to use the flat AP ones more often, but I use CDR when we don't have enough CC on our team.

For Quints, I use flat AP. I don't really like flat health because that 15 AP really helps for early game harassment.

Starting items:

For items, I start out with an amplifying tome because A. you get more AP B. you can buy a health potion. You can also go with Doran's ring, however I tend to not get Doran's ring most of the time. The reason behind this is because with Amplifying Tome, you get a little bit more AP, and the ability to heal via health potion quicker than you would regen health from Doran's ring.

I find that the quick health regen is extremely important due to the fact that your opposing lane partner's harassment ability CD is probably rather short and healing quickly is a must.

If you choose not to get Teleport, then by all means, take a Doran's ring, because you will be spending more time in the lane and you will have to depend on health regen to regain health.

Boots:
Please please please DO NOT UPGRADE BOOTS RIGHT AWAY. I cannot stress this enough and I see many people doing it very often. If you spend that gold on an AP item instead of upgrading boots, you will ultimately do more damage. Reason for this is mainly because at this point in the game, you don't have opponents with high magic resist so penetrating 20% of their MR is negligible.
Example: Most players, unless they are tank (even so, they probably don't have MR this early unless you have an AP heavy team.... or facing a trundle).

For the rest of the items, they are pretty standard AP Carry items. I choose to get a Rylai's because of your E. You cast E, it slows (Rylais does NOT add additional slow at this point), detonate, and then they are slowed. The slow is pretty minor because you only get 15% slow due to only having AOE spells. I like to get it because of the added health and AP, the slow is just another good thing about it. Feel free to replace it.

In general, I choose to get more damage items rather than survivability items because lux has pretty good escape (if you miss your Q, then you're done for). If your snare wasn't enough, toss an E to where you are at the moment so that they receive the full duration of the slow, don't bother detonating either unless they're about to die.

This brings up another point where some might say "snare only snares two people, what if you're running from more than 2, wouldn't you wish you had more survivability?" My response to this is that if you're in a situation where you're running away from 3+, chances are that you're doing something wrong. You should ALWAYS be near your tank (tanks ALWAYS have CC and can help you flee). However if you wish to build survivability, go for it, just don't forget that you will have to sacrifice damage for it.

NOTE: This may be obvious, but for some maybe not so, please please please do NOT get an RoA at the same time you would your Rylais because 9 times out of 10, the game will be nearly ended by the time you reach a fully grown RoA. If you want to purchase this item, rush it.

This section is rather simple.... toss your E to slow them, when they are just about to get out of range of detonate, snare them, autoattack, detonate, autoattack, and ult if they are low. Lux can't really kill many with one combo (unless you're against another glass cannon). Don't forget that your ult does massive damage when it's rank 3 and when you have most of your items, often times it will knock out half (if not more) the health of most ranged AD champions.

Once you're level 16, feel free to spam your ultimate when it's up. It is ~24 seconds CD with blue buff (which, you should ALWAYS have).

Tips on landing Q:
It's a skillshot, therefore it will take practice. All skillshots are different, some travel fast, some travel slow. My only suggestions for landing this spell is to cast it where your target will be in the next .5 second. Also try to always be in diagonal range of your target:
Always toss it a little bit towards the direction of the target. Your snare is not a thin straight line, it has some AOE to it. Tossing it like this will increase the success of your skillshots.

Tips on landing E:
Walk towards your target, if they run, toss it where they will be in the next second. If they stay/fight toss it, run (they will most likely chase for a second), detonate, and autoattack. It has a pretty large AOE, you shouldn't be missing this.

Tips on landing R:
This is the most difficult skillshot to land on lux. Master this, and you master Lux. Since there is a bit of delay before casting, and can be dodged, try to cast it like you would the snare. It has a bit of AOE to it as well so casting it diagonally will increase your success. Don't be afraid to cast a blind ult. If someone is very low, and you're 100% positive your ult will finish him/her off, aaaand you lose vision (fog of war), cast it in such an angle that you will hit the turret they are hugging to recall. Blind ult requires a lot of practice, but is not impossible to master.

Clarity because if you spam your spells (which you should be doing), then you go OOM ridiculously fast. Also, this allows you to stay in your lane longer, so you don't have to recall because you have no mana.

Teleport because you need to be where you're most needed; whether that be to grab a kill, save a teammate/tower, teamfights, dragon, baron, etc. Don't forget that you can teleport to wards.

I supposed you could swap teleport for flash/sprint (not both) if you wanted to but I don't think Lux has any trouble running away. If you're choosing these spells for chasing, why not just ult? Ultimately, it's all about preference.

Other Summoner Spells:

(*)Revive - stop trolling.
(*)Heal - STOP trolling.
(*)Rally - STOP TROLLING.
(*)Smite - Leave this for the junglers.
(*)Clairvoyance - This is very viable, however requires a good team. Good in ranked.
(*)Fortify - Not the best choice for Lux, because the +9 to minions sort of goes to waste. I'll leave this up to you.
(*) Cleanse - If the other team is very CC Heavy, get this.
(*)Exhaust: - Whenever I play Lux, I try to stay as far away from everyone else on the other team due to A. I'm totally squishy B. All my skills are ranged. However, if you need it for your team (because they have Trynd, master yi, gp, etc.) then feel free to take it.
(*)Ignite - Not terrible on Lux, adds to her combo, but I don't think it's the best choice.
(*)Sprint & Flash - You don't need another escape if played correctly/safely.

IF A. You have Blue Buff B. Your ult is lvl 3 C. There is no teamfight soon,
feel free to use your ultimate to farm that huge minion wave

Please leave comments and opinions and don't rate before you try =)