You need to use you W generally how you can. For that you need Tear of the Goddess, and stack this. You also can use your Q, W, R and E to do good damage. Ignite is too useful.

Nami can take good ganks, because she has good ult and Q for this, also speed and heal.
Try to no miss your Q.
You need some practice to play good her on mid ;)

Other spells are no so good for Nami on mid, she need ignite to hold more damage ;)

She hasn't hard combos.
Q + W + R + E
R + Q + E + W
You can flash before your enemies, stun they with Q and R, and kill them with your team.