Hmm... I'm probably not the best person to give advice on this topic, as I am a compulsive side-torso killer. In general, though, unless they have a large or easily hit CT (Daishi, Madcat, Sunder) I tend to try to shoot out a side torso first, preferably the one with the most weapons. For example, several mechs have a two beam slot/two ballistic slot in their right torso (Shadowcat, Thor, Highlander), and so those make good targets. That really only applies to fresh 'mechs, though. If the 'mech has taken any damage to its torso areas already, I usually try to hit the previously damaged area. Also, for 'mechs with easily shot out heads, like a Madcat or Daishi, just aim a little high for that CT shot, and you've got decent chances of a pair of consecutive headshots. If not, their CT will be heavily damaged anyway. If you know someone has weak leg armor, or if their leg is already flashing red, leg them. If you see someone with mostly full torso armor but they're about to lose a weapon from one of their arms, shoot the arm off. The most important thing is basically to aim where they're already hurting the most. Watch carefully for people who don't armor a certain side torso (for example, the torso on the hauptmann that has the missile rack is frequently not used, some people strip significant amounts of armor off of that torso). Black side torsos frequently will give you a kill if you put one or two well-placed shots there (depending on how much you're carrying). Also, if you don't have a mech on your radar, but you see one of its torsos has sparks flying everywhere, shoot that torso! Nearly any weapon will kill a mech at that level of damage. One last thing- if you alpha a side torso when you wanted to hit the CT, don't switch back to the CT, keep shooting the same side torso. Rear torsos and headshots are certainly the quickest way to kill someone, and center torsos are the second quickest (unless they're packing light leg armor), but I just take some perverse pleasure in shooting out side torsos.

Not to say that it's terribly slow, it usually doesn't take much longer than a CT kill, plus you get to knock out weapons along the way. It'll just be your own personal preference how you like to blow stuff up. I hope this helps, but there's really no substitution for experimenting with various methods.
Hmm... now that I'm thinking about it, side torsos might actually be just as quick for killing as center torsos. Each side torso has a set amount of internal structure which is not viewed as part of the armor. The center torso is different, because once the armor there is gone, you get the "damage critical" warning, and then they have to shoot through the rest of your internal structure, which is viewed on the health bars as armor (flashing red = actual armor gone). The side torsos also have this internal structure, which is similar to the amount of internal structure on the center torso, except damage will transfer to the center torso even as the internal structure of the side torso is being destroyed. And once the internal structure of the side torso is gone, the mech is dead... Well, I don't know if all of this paragraph is completely accurate, but I think it's pretty close to the actual kill conditions. Anyway, I'll go experiment with it. Or maybe the damage transfer is going to the internal structure of the center torso... that would make sense, because the armor color for the center torso remains the same color even as it goes down from damage transfer.

I have a feeling I just stated the completely obvious... Oh well, I really need to stop rambling...