This is how I got good at last hitting with Anivia. Getting used your different champs auto attacks really helps, especially when it comes to champs like Anivia and Fiddle, who each have terrible auto attack animations.

You can't master it. You can only get better and better.

It highly depends on what is happening. I have hit +50 creeps more than my whole team really early in the game because we were a Graves Soraka lane, thus we shoved out the enemy while I never had to go back since I never really got that low.

I always CS till I am forced to go back (alias low on health, no wards etc.)

Please don't give such a smart ass remark. Last hitting isn't just based on waiting for it to be low, then attack it. Alot of people misjudge the creeps hp and hit a tad bit too early and lose the CS. Also, if you don't have the proper positioning you may take some harass from the opposing team. If you can farm under the tower by knowing the combo of [Hit] [Tower] [Hit] and [Tower] [Tower] [Hit]. Knowing if their are two minions low to hit one with a basic attack and/or use a spell to catch the other minion.
All of these play a great deal into last hitting, so next time don't be such a smart ass, and perhaps if you don't understand the post he has made, then he isn't looking for your input as it was invalid/useless.