As soon as cassi hits level 3, your burst is nuts. Use your Q whenever you can for farming and harass. Whenever you poison an enemy always hit them with your E if possible. Q and E are the extremely easy to spam, and for this reason, you need to put your E on smart cast. Doing this will allow you to Cast your E that much quicker, often times allowing you shoot an extra E or maybe 2. You can clear waves really quickly, so use that to your advantage. Cassiopeia can also trade really well, because she can get 5 hits in the time it takes others to get 3. Ideally you want to be aggressive early and assert your lane dominance.

Cassipoiea is an exceptional ganker. The speed up from her Q and slow from her W make ganking a breeze. Add her ult on to all of this and you're sure to get the kill for the lane. Make sure you take the wraith camp whenever you can, and ask your jungler for the blue buff after he gets the first one. Once cassi gets blue, that on top of her passive will make your mana pool seem endless. The general thing you want to do with cassiopiea is cast your W and a couple of Q's to push the lane, then go and gank other lanes.

Cassiopiea brings a lot to the table in team fights. Her ult and constant poison damage can decimate an entire team in a matter of seconds. Keep in mind, however, that you ARE one of your teams carries. Do NOT under ANY circumstances put yourself in 1v3 situations because your damage is high enough. 3 under farmed CC champions WILL be able to take you on, no matter how fed. Stay with your team. Your ult is such an amazing tool, because it allows your entire team to have a break from the enemies damage, all the while dealing damage of their own. This completely turns fights around and can turn a 4v5 where their team has baron, into a fight that your team goes 3 for 4 in and wins the exchange. Don't be afraid to ult smaller groups, your ult IS an amazing tool, but it isn't something you should be scared to use. Your 3 other moves provide enough damage where you can still bring something to the table for your team when your ultimate is on cooldown.