Oh man, I could go on and on and on..
And to think I actually thought that going to art school was a big waste of time, I even considered dropping out! I don't think I learnt as much from my teachers as I did from my classmates though. There were some really skilled guys in my class, and it seemed to be easier to learn from them than from the teachers.
That and the fact that most of the teachers would just say "Observe carefully!" Nothing else :P
But I digress. There is a lot I learnt in school which has helped me in a BIG way
Ok first, the thing that helps me SO much: negative space!! Seriously, I cannot stress on how important it is to take it into consideration while drawing/painting. This is what helps to bring accuracy into the drawing, and it is not even complicated to look out for! It's even easier when you're working from a 2D reference, as opposed to drawing from life.
Then, squinting. Hehehehe. Well, it really helps to eliminate the unnecessary details and concentrate on the form/mass. I just-could-not remember to do this though. I have actually written "SQUINT!" in big bold letters on my easel just so I would remember :P Like all newbies I would jump to the detailing as fast as I could, a big no-no. It's easier with the digital media though, no need to trouble your eyes-just apply a blur filter :P
Another thing about drawing from life, is the proper positioning of the easel. It might seem silly, but there were people in my class who would turn about 90 degrees between the easel and model. I think if I tried that I would forget what I just observed while my eyes reached the canvas. SO, I learnt to position the easel in a manner that I could look at both the canvas as well as the model simultaneously, it's easier to compare and fix errors that way (something like what MechaHateChimp said). Also, I learnt to stad as away from the canvas as possible while painting (which would mean, at the length of the arm+brush). This is because the closer you get to the canvas, the more you tend to detail and forget the big picture. It can really ruin a painting. The digital equivalent to this would be to zoom in and out frequently I guess.
Also, I think I really understood something about colours only in the third year, We had to paint a figure with a bright green background. So we were instructed to prepare a ground of the same colour (value+hue+saturation) and then paint the figure on top of it. It was the most frustrating thing ever, every colour one prepared on the palette (which was white) looked SO different when applied on the bright green. That assignment went terribly, but I learnt
Hm, another thing that I hated at that time, but which in retrospect seems like good training, is how we applied colour. I think in most schools they teach the layered method, where you have one layer for the drawing, one for the underpainting, then the greyscale and so on. Well, we had a week for each figure painting, out of which we would lose 2 days in general dilly-dallying and cutting and stretching canvas etc. So anyway, we had 3 days (15 hours-ish) to paint a figure in a 30" x 36" space and the only way to do it was to apply paint(oils) in patches. So it was wholly alla-prima and while it is a super-frustrating way to work in, I think it manages to teach one a lot in the way of observation.
Here's a sample from 2 of my old paintings*cringes* Ugly stuff, but you get the general idea
Edit: One more! I think it is important to not restrict oneself to one particular style of art. My electives in my final year were Portraiture, Non-representational painting and Printmaking. A lot of people find that a very strange combination, since Portraiture students usually take up Representational painting. But I think I learnt SO much more about the nuances of colour and texture this way. I studied a lot of Rothko, Pollock, Gaitonde and other Abstract Expressionists and I know it helped so much.
There's an interesting quote I came across a while ago by Marlene Dumas:
"What a funny thing painting is. The abstract painters always insist on their connection with the visible reality, while the so called figurative artists insist that what they really care about, is the abstract qualities of life."
Sorry for the really long post!I think I went a little off topic :P