Focus a lot on minions. Best way to push a lane is to CotV the rear minions and put down a MV on the weakest one. If there's a turret minion, nail him a bunch so that your MV can take him down when it lands on him. Anyone you're up against won't want to get near MV for fear of any followup combo or gank.

Get Call of the Void first because you don't need a target to land it (Teemo, bushes, etc..). It's blind castable and can be spammed up at the fountain for your free voidling.

Then, focus on Malefic Visions, levelling CotV when you can't. Don't get Null Zone until 12 (last possible). It's not that you don't want it's damage and zoning potential, but focusing CotV and MV allows you to maximize damage output. With the low cooldowns you get, you'll be a very busy clicker.

Remind your teammates that your ult is gonna be on a very short C/D.

Try not to depend on your ult for damage so much as fight control. People ***** when you lock down a tank or CC, but losing a decisive tank can really turn a fight around. Put down your Null Zone to protect an angle while you lock down.

Sounds silly, but if your teammates are onboard, it will work very well and the enemy team will fall behind their towers, where you can pick it off.

Figure out when someone has you pegged, and kite them hard. Remember, this game is about towers, not champ kills - that's how you win. So use Malzahar's great range and spammability to get those minions out of the way and champs on the other side of their towers, afraid of you.