Last Hitting Advice

So, after thousands of games, I feel like I'm pretty good at last hitting. However, I usually play mages so I'm used to having abilities to help. I find when I get pushed to tower, or when I have particularly weak auto attacks (Fiddlesticks) I have a lot of trouble last hitting. Why?

HOW THE FUCK DO MINIONS DECIDE WHO TO AGGRO?

I cannot figure out the minion behavior for the life of me. For instance, one particular one that pissed me off the other day; Pushing my lane, waiting patiently for a ranged minion to get down to enough to last hit it. It was the cannon minion wave. The cannon minion was attacking a melee minion. Without warning, it turns to the ranged minion, which I was about to hit to kill, shoots it, kills it, then goes back to the minion it was just attacking. This shit drives me insane! Is there some secret to making minons attack who you want them to attack or something?