Land your Q correctly. I know you said don't say it, but Cass's Q is all her damage. It allows her to follow up with her E, it is her safest/quickest/best harass in lane and in a team fight and it has an amazingly short cooldown.
As far as Q goes. Aim to where they're going. Cast it as if you know where they're going, don't aim it where they might not be in the next second. Also learn to cast it close to yourself, because I can't count the amount of times I've hit good Q's from afar, but failed to hit a proper one close up when theyre running at me.
For her ult, don't use it too late. The slow is pretty heavy, but you want to make sure you get the damage and that useful stun to get a guaranteed Q+E+E+Q off in the time period. It should almost always guarantee a kill in lane. Don't harass them to 1/3 HP then use it as they run. Instead use it when they're 1/2HP if not more since you'll get the damage burst and the stun, though you'll need to be somewhat unpredictable about it otherwise they'll guess exactly what you're doing.