So i have been playing Cassiopeia for almost a year (approximately 200-300 games) now and i have built a system that tends to work for me.
This guide is based on extreme aggression and caution in middle lane on Summoner's Rift.
You will be pushing as much as possible while avoiding ganks from any junglers that may want some snake in their diet.
This requires a lot of skill to do and you need to rely on champions to call any and all mias so that you don't get killed right away.

The Start of the Game:
First off, with the runes and masteries set in place, you should begin the game with 35 ability power with Doran's Ring and take Noxious Blast. You quickly make your way to the middle lane.

With this build, you are extremely strong against a few enemy champs that could be in your lane. If you harass enough, enemy champs with a natural heal will not be able to heal fast enough, even with runes.
Against Fiddlesticks, try to avoid all drains that he may activate on you. Draw his attention away from you and make him heal off the minions. When he is standing still using Drain, drop a Miasma slightly behind him and hit him directly with a Noxious Blast. This will reverse the healing he tried to do and as a bonus, he used Mana and his ability is on cooldown.
Against a Vladamir, he will use health to do damage and to gain health, this is perfect for Cassi who drains health like crazy. Just keep hitting him with Noxious Blast, and eventually he'll stay far back avoiding your quick poison shots. This will give you the experience boost you need to out-level him early game.
If you are against an enemy Teemo in mid, you have a big advantage against his blind dart. Since it only affects physical attacks, Cassi is unharmed by this spell as she used mainly poison spells to her advantage.

If you are playing against an enemy who stacks up on health potions, you will quickly realize how fast he will use them if you keep the aggression and hit your mark with Noxious Blast whenever he is open.

Cassi is also good early game against mid tanks like Sion and Alistair since they are melee champions. As you stay back and poison them, they will become frustrated by you and try to attack as you retreat towards the turret in defense.

The second skill you should get is always Miasma. Because noxious blast works in a small area, an enemy who does not sit still will be hard to hit. When you toss a Miasma down, this will slow the enemy enough to hit him with a Noxious Blast or even a Twin Fang.

The third ability you get is usually twin fang or Noxious Blast(2). You choose Noxious when you think your are going to continue successfully hitting the enemy champ when they come close. You choose twin fang if they are a weak health opponent like Vayne and Teemo. This will surprise them when they come too close for comfort and may cause them to flee seeing that it has a large damage output and a very quick cooldown if your target is already poisoned.

Once you kill an enemy champ, clear the minion wave and quickly head back to camp. At this point, you should have enough money to buy boots and a few health potions, and maybe even some sight wards. If you have enough, buy an Amplifying Tome as well, making your poison even more affective towards the enemy in lane.
Once your back in lane, your opponent should be there too with a couple of health potions. (you will laugh because if they didn't have any before, they should now know better to get some at this point). Since you want to avoid showing that you place wards, go to the sides of the river where it is still land. Place each ward over the wall and into the grass. This will seem like you are going to gank bottom or top but you "changed your mind". Then when an enemy comes to gank you in mid you will see him clearly. An advantage to this is that you can throw a Miasma over the wall and hit them with a Noxious slowly draining their health and giving away the element of surprise they had planned for you. If they are an enemy like Xin Zhou, if they briefly hesitate for less then two seconds, you can pop your Petrifying Gaze over the wall, either freezing him or slowing him in the spot, giving you the ranged advantage to kill him.

If you notice that your opponent will not leave the lane no matter what health they have, you can always flash in and pop ignite and a Noxious Blast to finish them off. This can be done even at the turret at level 5 but may require you to retreat to base after.

Attacking the Enemy Turret in Mid:
Once you send the mid back, this should give you the open advantage to attack the turret with your auto-attacks. This is the opportunity to regen your mana, you've spent on harassing and over time should wither away the turret's health. If you're lucky, it will be destroyed after 10 minutes. If your opponent is putting up a fair fight, it may take you up to 20 minutes to kill it. You can also attack it if your laning enemy is trying for a gank at top or bot.

To escape the enemy:
There is always gonna be times where you are surprised by a jungler or a missed MIA call that has come to gank you at mid. This is why you have Flash and your ultimate. If you can flash away from your opponent, do so to get as far away from possible. if they use a slow or Ghost to continue to chase, use your ultimate behind you to stop them in their tracks. If the opponent decided to attack you at a reduced health, your ulti should do solid damage and your poison should quickly melt away his health. If you have enough ability power when they try to gank you, your noxious blast alone should drain your opponent with ease.
Another advantage to this is that you may be a higher level then they are because of the soloing you have been doing in mid. If your Noxious blast and Fangs have been upgrade enough, the fangs should do devastating damage to the enemy after they are poisoned and quickly doing the same damage again instantly with the reduced cooldown.
Don't forget that if the enemy is going to dive the turret, your ulti plus poison will do some interesting damage to the enemy that WAS hungry for the kill.

Planning on having your teammate gank at mid:
If the jungler on your team is ready to get a first kill, you will have to pull back your punches and think they have the advantage, if they are close to the river, flash in and use Miasma or your ultimate to slow them, this give your ally the chance to come in and do some serious damage, if the enemy uses flash, the poison you have done should be enough to kill the enemy unless your ally flashes for the chase. (Note: Feeding your ally junglers is GOOD)

Team fights:
If there is a team fight in mid, you want to protect your turret and destroy the enemies. You will need to be semi-aggressive but also passive. Your tanks or initiators should go in and take the damage that would be devastating to you while you hang back and put down Miasma and several Noxious Blasts before spamming the Twin Fangs to do serious damage. Your ulti can be VERY useful in team fights where there is a lot of melee champs. If you notice that your ally is low health and being chased, everyone's first thought is that they should save him. Use your ulti on the enemy chasing your ally and unleash as much damage as possible. After the 2 seconds, your allies should have wiped out the opponent that would have killed your ally.
With Cassi, it is important that you pick off the low health champions first. Two blasts of poison and Twin Fangs will deal enough damage to wipe them out quickly leaving you and your team to focus on the more stronger champs. Aim for champs like Lux, Xin, Teemo, Vayne, Kog, Sona, Soraka, caitlyn, veigar, and anivia who have even 1/2 their health missing.

Late game, Cassi is pretty much the support character, people would have stacked magic resist against you already and your attacks will do less and less damage. This is when you leave the battle damage to those who have high attack power. Your job late game is to slow and chip away health so your allies can take out the enemies quicker.

This is a very confusing section because i build differently each game i play.
You first want to start off with a Doran's ring for that extra damage for your poison early game.
Once you return to base, get sight wards, boots, an amplifying tome and some health pots.
Once you get enough money, you buy a Needlessly Large Rod and after 2 Blasting wands. Then your build Rabadon's Deathcap and void Staff.
At this point, your AP will be about 200 and doing serious damage. Getting these two items is about average for a Cassi game. At this point you want to either choose one of three ways.
1. If you are getting hit by some serious damage from leblanc and gangplank's criticals, you may want to build health. In this case, get a Rylai's Crystal Scepter.
2. If you want to keep up with the devastation, get another Rabadon's Deathcap to increase the damage output you do with all your abilities.
3. If notice that you go in too much and get focused down before the teamfights start or being targeted by that one champ who won't let you live, build the Zonya's Hourglass to keep in the fight for a few seconds longer while your team helps you kill your nemesis.

Build the rest of these after the others according to how the game is going. if you need to build serious magic resistance, build Wit's End after the void staff instead and read the game according.

This is a list of a Hints and Tricks i use with cassi to avoid danger, get kills, disrupt plans and have fun.

-Your Miasma reveals people hidden in bushes up to a certain radius.
-Get Blue Buff before any major team fight.
-"KS" can mean Kill Secure, it is a team game and giving certain people the kill is good but if your enemy has the potential to get away, don't let them, it'll only cause problems later.
-You can wipe out huge waves of minions in a mere 2 seconds with just miasma and noxious blast where you can connect the majority of the minions within the area.
-if you do /t when your facing a wall, it will seem like your crawling out of battle.
-if you flash into the bushes, move to the left and use ulti on person, they expect that you are fleeing, but in fact, setting yourself up for the perfect opportunity for your Petrifying Gaze which then you can unleash devastating damage or give you enough breathing time to retreat faster.
-when akali puts down her invisible shield, you can throw poison into it and it will still do damage and annoy her. Using your ultimate will do the same but will cover the entire area and potentially freeze her. If her shield runs out just as you use it, she might be caught in a very dangerous position.
-i don't know if this works, but trying to freeze Nocturne as he uses his ultimate on you.
-throw down poison to slow enemies that could potentially open up a team fight.
-keep moving when playing against champs like Nidalee, Lux, Kog, Xerath, Amumu and Leona.
-if you see teemo go invisible keep hitting that spot with poison and hit him with a ignite if he comes out to run.

This will be a guide from people who have given me advice to use certain items at certain times:
i will post the results of the builds here:

My ID is HarbringerXL.
I play champions like Lux, Annie, Kog, Wu-kong, Teemo, Nunu and Ryze.
My best is Cassiopeia.

This guide is my way of playing the game and hopefully it would be helpful to you guys who want to play Cassi as another Champ.