You want the wave near your turret so she can't run you down. Try not to push the wave when you are farming with q.

Max Q>E Focus on dodging qs. She does no damage with her q if you are not poisoned. Don’t trade with her early or try to cheese her with lethal tempo in lane. You can’t stick to her and she has more sustain than you. Go Dshield and second wind for sustain. Try to be unpredictable with your movements so she q’s early. You can cancel autos when last hitting and walk away so she is baited into q’ing when you last hit.

Do not trade with Cassiopeia early on. After six you can look for autos on her when her q is down.