9x magic pen marks - best in slot
9x mana regen/lvl seals - best in slot; cass needs the mana since her abilities cost quite a bit of mana
9x flat ap glyphs - my personal favorite; i feel casseiopeia's early game is ridiculously strong. you can harass heavily against almost any champ in a solo lane. may be replaced with ap/lvl glyphs
3x flat ap quints - same as the glyphs; makes cass' early game ridiculously strong!

9/0/21 is the standard AP carry build;
archaic knowledge for magic pen is a must as are the attributes given in the utility tree.

flash/ignite.
flash needed for mobility purposes. ignite to rape early and mid game. crazy damage along with cass' damage over time abilities.

max out noxious blast out first. your main source of dmg. max out twin fang next, taking a point in miasma around lvl 5 or 7. this will help you clear creep waves easily with noxious blast. pick up petrifying gaze whenever possible, obviously.

start off with a doran's ring or boots and 3x health potions. on your first trip back, pick up an extra doran's ring or two if you are having difficulty farming; otherwise pick up an early catalyst. upgrade to sorcerer's boots and finish your rod of ages. cass needs the health and mana pool as well as the +80 ability when the item reaches its potential. pick up rylai's crystal scepter next. this item is fantastic on cass, allowing her to slow enemies with her ability, making it easy chasing them down for kills or kiting them to get away. with these two items, cass gets 3000+hp, giving her plenty of survivability. next, get rabadon's deathcap, the most powerful AP item in the game. finish off with 2 of the next 3 items: void staff, abyssal scepter, or zhonya's hourglass. void staff gives you the AP and magic pen. abyssal skull gives you MR and reduces nearby enemies MR. this works with petrifying gaze AMAZINGLY. hourglass gives you +AP, armor, and the stasis effect that is useful in certain situations. check out your opponents' items and build accordingly!

1. Landing her Petrifying Gaze is critical.
Her ult is extremely powerful. Getting chased by 3 enemies? Run into a bush and ult them when they enter. They all get stunned and get hit heavily, allowing you to get away or saving enough time for your team to come in and clean them up. Make sure to hit as many as possible facing you. Use flash if the opportunities presents itself to position yourself to hit all of the opposing team.
2. Cass relies on damage over time.
Take note of this. Watch your dot from miasma and noxious blast carefully and practice estimating when an enemy champion will die from it. This will allow you to switch targets and focus on another enemy champion, possibly allowing you to help get kills that you otherwise would not have had you kept chasing an enemy that was already going to die.
3. Cass' early game is ridiculous.
HARASS HARASS HARASS. Her early game damage with flat ap quints and glyphs is absolutely crazy. Learn to hit noxious blast and quickly following it up with twin fang for a couple extra hits of damage. Remember that twin fang's cooldown goes to 0.5 secs on an enemy champion effected by noxious blast or miasma. Use ignite when you're confident in for a kill. You will dominate your lane against most champions.
4. Get blue buff whenever possible.
Have your team give your blue buff to allow you to spam your abilities and to boost your AP damage. The extra duration from neutral buffs in the utility tree benefits greatly.
5. Drop miasma wisely.
The poison damage is crazy and it also slows. Drop it into spots where the enemies will stay on it as long as possible. In team fights, drop it where multiple disables from your team will keep them still. Another area you should try to put it is on your teammates that are being focused, especially if it is your tank. If the opportunity never presents itself, drop it on yourself or your team's carries to help making the opposing team weary from jumping on you.