Runes

The Runes in this build are pretty straight forward for Caster Carries.
Marks - Flat Magic Pen, These give you additional Penetration ontop of the tons you will have in each build, and help you quite a bit early since you don't get much AP until Mid game.

Seals - Mana Per Second/18, These allow you to spam your abilities a bit more safely when Clarity is on Cooldown, Since she still does destroy her mana pool with a build like this

Glyphs - Flat Cooldown, These allow you to use your Q and E abilities more freely, as well as those fun Double Rainbows.

Quints - Flat HP, Since you get boots first with this build the 78 HP is VERY useful early game. These can be replaced for AP quints due to the HP reduction Patch for Masteries and runes.

Lux's Passive is not to be overlooked. If you can get in one auto attack before they turn around and try to hurt you, do it! (as long as they don't ACTUALLY hit you) This can be great for last hitting minions, annoying enemies, and getting those kills on enemies running away.

Q
Lux's Light Binding is very useful, and her main damage move besides her ult. If you hit an enemy with this you can set up perfect ganks, kills, or just scare the hell out of them. Aiming it can be bothersome though as it DOES NOT PASS THROUGH MINIONS so think before you shoot this. Wasting mana on this besides to hit champions is not recommended.

W
Lux's Prismatic Barrier is quite overlooked though since most team fights don't happen until late game, you typically wont even need to look at this until nessicary. I've killed 2 people before with 400 HP left, ran to heal and saw Karthus's ult coming and have saved myself many many times with this. It can also shield allies as well and can help your team that much more.
E
Lux's Lucent Singularity is her main farming move. This can also be used very strategically with Light Binding. You can heavily null an enemy from running, force them to get hit by it or run into a gank, allow yourself to hit your passive 2 times in a row and more.

R
Lux's Finales Funkeln is not to be taken Lightly, with this build you're hitting for over 1k damage every time you use it to every champion or minion you hit. add that with it's super low cooldown and it's a force to be reckoned with. Combining this with your Light Binding and Lucent Singularity can cause extreme havoc to the enemy team. Also, don't be afraid to use this a little more than you want to because of the cooldown it shouldn't be a hazard if you do.

Your teammates will love you for your Q, because that is what will get them the easy kill any time an enemy champion steps in your line of sight with your partner nearby. As I mentioned earlier, use your Q to trap enemies who are in the wrong spot at the wrong time (like under your tower, trying to run away, or trying to join the fight while you're focusing someone else). If a teammate is being chased, run to them and throw your Q at them to stop the enemies behind them - and immediately also turn tail and run.

The early point in E is so you can do additional damage with your passive while you wait for your Q to cool down. One of the big advantages of your E is for free sight. It doesn't have the range of Ashe's Hawkshot, but it is still plenty to do from a safe location behind Baron, Dragon, one of the buffs, or (typical case) into a bush on the side of your lane. (Even if just the side of it grazes a bush, you can see inside it, plus enemies don't always realize that this ability lets you see them.) Use your E to see when Baron or Dragon is close to death, then hit it with your laser to steal it - very pro, since you'll hit their team while you're at it. I don't often find much use for the slow effect because of also having the snare, but that's just me. Every little bit helps when you are trying to run away - throw it just in front of you or your teammate and leave it active at least until the enemy is almost completely through it.

The early W point is because you will certainly have time to spare during any fight to throw your allies a small shield against any damage. The question is whether you will have mana. Therefore, laning with someone who has clarity is a plus. Late in the game when you finally start putting levels into your W, it can save you and your teammates from things such as Karthus' ult (throw it as soon as you see the red line appear above your heads, and move to make sure your allies get shielded) and poison. It can also help turn the tide in a teamfight especially if the enemy has some AoE damage and your team is clustered together.

I mentioned that you will be loved for your Q, but they will all bow down and worship you for your laser. As with all of Lux's abilities, this takes practice in aiming it just right. Very often I get kills by shots in the utter darkness, because I correctly predicted where the enemy was running. I was very confused when Caitlyn arrived on the scene and people commented that there was finally a sniper - hah! If you want to call yourself a sniper, try aiming instead of just choosing your target. Anywho, another tip for using your laser that I mentioned earlier is, after you've maxed it out (or at least gotten your second level in it plus a few CDR items), fire it right away during a team fight and it will cool down by the end of it - but make sure that first shot hits as many champions as possible. Also, because of the low cooldown, I like to fire it before I leave a lane to clear out extra minions and scare the opposing laners from getting too close to my tower because they are now at half health.

Finally, something I should mention (and reiterate if I've already mentioned it) is that Lux should not be at the forefront of any fight. She is not a tank. She should always have [at least close to] the lowest damage taken at the end of the game. The HP regen items are solely to keep her in the lane earning XP and GP between TPs. If the enemy is focusing you, you're either playing too far forward, or your team needs to watch their flanks better before initiating team fights. If they do start to hit you, snare them and run away. If they're going to kill you, just laser them ASAP to do the most damage before you bite the big one - it'll recharge by the time you respawn anyways.

Do NOT spam your E while laning or you will have major mana problems the whole time you do that. If you find yourself running low on mana, STOP using your W and E. You always want to keep enough mana for your snare in case you need to either get away or trap a foolish enemy. Use your E strategically - place it in front of a minion wave that is on its way to your tower to slow them, and explode it at the last moment to get most of the minions.

When you are just clearing a minion wave and you don't have to worry about saving your mana, go ahead and use your E, your W, and even your Q to stop them from advancing. Unless your ult is max level and you're not hurrying to a possible team fight, don't bother ulting the minions even though it will take a good bit longer to clear them. With your teleport, you should be keeping an eye on the lanes and clearing them before the huge wave reaches your tower (especially after they just got buffed!), thus using your TP to get back into the action rather than having to TP to your dying tower.