This is my first guide on Mobafire, let' hope I get above 50% :S. My english isn't all that great so feel free to point out mistakes in the comments. Lux is one of my favourite champions.
Both are VERY flexible. If you play with a lot of CDR runes you won't need cdr items. People have different playstyles, I like sustainability so I go with fortitude quints and cdr in items. Really it's at your discretion.
Pretty standard here, 9/0/21.
Lux's ult scales exceptionally well with CDR, going down to 24 seconds with 40% cooldown. Lux loves golem and ask your neighborhood jungler for it whenever available.
Lux I find to be an exceptional mid champion. She is VERY mana intensive so don't spam abilities. After the Morello's Tome you should be fine casting Lucent Singularity and last hitting creeps with Lux's amazing passive. At level 18 with your 24 second ult feel free to farm lanes.
I tend to deal large single target damage so I max my snare first. I do take a point at prismatic barrier at level 5 to maybe protect from a stray ignite. The Combo I use most in teamfights (assuming a mildly well farmed level 18 Lux) is Q>E>R>E>Q.
Although this guide is focused primarily on damage, Lux still needs to support. All of your moves are amazing in a teamfight situation. A well placed prismatic barrier can give everyone on the team an extra 600 health. Your Q can disable the enemy melee carry and the E prevents escapes with the extreme slow.