Every champion is different when it comes to last hitting, and some are easier purely because their AA hits harder then other champions. This doesnt make CSing under tower any easier. And if you don't know the champion well enough it'll make it even harder, specially if you play as Anivia who is super slow and doesnt hit very hard and then try to CS with Orianna who AA hurts and has average AS.
I think it comes down to 3 things;
Experience, Knowledge of Champion, Patience
Experience of how a tower works is easy enough, you can get a perfect CS score even under tower if you know how to use the tower to your advantage.
Knowledge of Champion, takes time and games. Any champion can be good at last hitting under a tower if you know how much damage your gonna do and when to use that damage.
Patience , dont get flustered that your under your tower; calm down think, plan, and click precisely. Use of the S button never hurt either.
Also if your being supported, how well your support can help you last hit under tower can really help. As a support I will typically hit minions to make it easier for my Carry to get last hits, although this only works if your Carry knows what your doing and times their attacks accordingly.