You can substitute Void Staff for an early Rod of Ages, then you'll be more durable but hit lower and waste some money. Also try to stay behind in team fights and deal serious damage when the team fight has started.

Mejai's Soulstealer is a good item to purchase instead of Rylai's Crystal Scepter or Void Staff but only if you are dominating the enemy team.

Cassiopeia can deal big damage to the Dragon so don't hesitate to kill him when you can, but make sure sobody else is tanking and also you can solo the Dragon if you got Will of the Ancients.

If the enemy team is too fast, substitute Sorcerer's Shoes for Boots of Swiftness

Kage's Lucky Pick is for not getting underfeeded. If you and your team are dominating then you don't really need to buy it.

Having 5 mana regen / 5 seconds in early can lead you to a good farming on minions with Noxious Blast.

You can go 3x Greater Quintessence of Magic Penetration or 3x Greater Quintessence of Replenishment depending if you want to farm more or gank/kill the enemy champions a lot.

Flash is best since Cassiopeia isn't a good runner and also can get you out of ganks.

Clairvoyance is a very good skill and you should have it if you're playing with Cassiopeia

Exhaust is a good skill to be taken by melee champions but if you like it, you can take it.

Ignite is a good skill to be taken if you are good at killing/ganking or the enemy has healers.

Ghost is not so good on Cassiopeia since if she's focused, she'll probably get stunned and you can't towerdive since you die easily.

Heal is taken only by newbies, but suit yourself.

Revive is the worst skill possible. No really, are you intending to take it?

Never Smite since you don't jungle.

Teleport is maybe usefull with a good team + wards but if you're going solo, DON'T take it.

Don't take Cleanse

Fortify is not needed. If you're a good Cass player, the enemy shouldn't reach your tower.

Clarity is not needed at all.

Rally is never used.

Petrifying Gaze can be an offensive and defensive ability. Don't hesitate to use it when one of your team members is being chased.

Your main damage comes from Noxious Blast and Twin Fang so try to stay behind as possible when your team pushes.

Miasma can be an offensive and defensive skill just like Petrifying Gaze but it makes an incredible combo with Twin Fang against a minion/monster or if the enemy is standing still.

Pros:
- Can deal lots of damage
- Good as nuker if you're good at playing her
- Very good harassment
- Can be a good farmer too
- Can slow/stun

Cons:
- Squishy
- Focused very often
- Can get easily underfeeded if you don't farm enough
- Extremly bad at running when she's chased

Here are 2 of my recent matches with Cassiopeia using this build: