Another good learning resource is this thread on the mwo forums: http://mwomercs.com/forums/topic/166699-hitbox-localization/
It will show you where the hitboxes are on mechs. I was surprised to find out how big the side torsos actually are on a lot of mechs. Also knowing which inner sphere mechs usually run XL engines is helpful to know when to aim their side torso instead of their center torso. I'm not sure what a good resource for that is other than checking the popular builds out on metamechs. Although if you're playing lights you're probably better off legging them like Flying Dice wrote.
Well, it depends. If you're running a close-range brawler light, absolutely go for legs, since you're guaranteed to always be able to hit them regardless of your angle of attack. If you aim for the body while circling, you spread damage across eight of their ten armor elements, and if you're doing <5 damage to any given component per shot it'll take a day and a year to get through a lot of stuff.
OTOH if you're running a sniper or a splat light, aim for torsos or cored components, since you have better control/higher burst.
Like he said though, you'll develop a feel for when you should XL-check an IS 'mech, and the primary tell is if they're carrying a lot of weapon tonnage for their chassis and still moving pretty fast. IS lights and most mediums will always run XLs, so if you see an open shoulder go ahead and hit that. Among larger 'mechs, certain ones (Jagers especially) almost always run XLs, while others (Marauders, most assaults) almost never do.
Also worth doing is leg-checking certain 'mechs, namely Jagers and Riflemen, because people like to use builds that strip a lot of leg armor from them - they're natural hill-humpers and normally shouldn't be exposed much below the shoulders. If one or two shots makes the leg armor go orange, it's a sure sign that they skimped.
Another tip, when trying to backstab in the CT, aim for the crotch. It can be tricky to land all of your damage if you shoot for center-mass but almost nothing has side torsos that reach that far down.
Plenty of other 'mech-specific stuff. If you see a Hunchback always try to shoot the hunch since that mounts most of their firepower. Same deal in general, there are a lot of 'mechs and builds that corner-stack most of their firepower into one side of the 'mech, if you spot one go ahead and carve that off. If you're trying to kill a Stalker, just fucking leg them, they've got a shitload of armor and structure, wonky hitboxes, and a body perfectly shaped for shielding (speaking from experience, I've driven certain of my Stalkers into brawls against 4-5 enemy assaults/heavies and come out with 1-2 kills while also living long enough for the rest of the team to show up and clean house). If you spot someone boating standard PPCs or IS lurms and there's not too much around, go ahead and rush inside their minimum range and fuck their shit up even if you take hits doing it. If you're driving a really small, fast light (read: Locust, Arctic Cheater, Spider, &c.), you can be a dirty bastard: if you spot an enemy 'mech trying to back up away from your team, just run up and stand behind him, as long as there aren't other people there to shoot you off of him. Don't even need to shoot, just be the invisible immobile pebble until he dies.
I mean, there's a mountain of little things to pick up. One I'd suggest is ignoring the armor spreads on Smurfy until you're comfortable with a chassis, a lot of the meta ones strip the backs super-thin - great if you can keep your back safe, but it sucks massively to go down to one shot from behind. There's nothing more satisfying than one-tapping someone with an AC/20 to the rear CT, and if you use one of those 4-6 rear armor builds without playing carefully you'll be on the receiving end a lot. I usually settle down at 8 (10 for assaults, excepting only super-lights like the LCTs that barely get 20 armor on the
front CT if they take 4 in the back) once I've got the skills finished and am comfortable with shielding on the chassis, anything less and a single gutsy splat light or well-positioned Gauss boat can ruin your day.