I’d love to hear some of moments that have changed or improved your work. Some examples might include:
well, I once spent a bit of time interning as a graphic artist for a small media firm who I drew some lessons from.
Firstly, I learned they had no actual relationship to a school of any sort, and didn’t know how to create a relationship. So, intern was pretty much a false title for the role. This also meant they had no understanding how to mentor, guide, instruct, or critique student-level work. I think this relates to the maxim, “Don’t take criticisms from anyone whom you would not seek out for advice.”
Secondly, the owner of the company was fully funded by a parent, and had taken no business loans, had not written and defended a business plan, basically didn’t have commitments to earnings; because, there were no debts to pay with revenue. They cut corners, didn’t work hard, didn’t charge competitive rates, didn’t write contracts for customer projects. Personally, I took the lesson that sometimes having financial obligations will by necessity define better productivity and better outcomes (but, not always).
Thirdly, they did not understand copyright law. They infringed on copyrights of legitimate brands, and got caught up in cease-and-desist and infringement issues that could have been avoided by listening to the employee who most certainly warned against taking the project with the customer, and/or writing contracts that outlined the requirements of licensing copyrighted media. I felt the lesson here was to be aware of the industry, listen to the folks you employ, and be certain to identify legal issues for customers even if that means denying their requests for content.
Lastly, I left to return to school for a completely other program. The lesson was to find the right community, get good advice and tips, and follow a path slightly more traveled. Although I know everyone will walk their own path, when others are sharing a similar path, it can be much better over time to have that community around for support, mentorship, guidance, and instruction.
My college design professor, Herb Camburn, who passed last year, once said to me, “If something sounds interesting, even if it’s not related to what you’re working on right now, learn about it. You never know when it will come in handy.” He was so right, and my head is full of the weirdest collection of stuff thanks to him.
Mindy’s are good ones!
A teacher of mine told me that grey is not a color. Neither is brown! So you have to look carefully to see the green, red, blue… that’s ALMOST grey or brown.
Guillermo Mordillo told me: Time doesn’t forgive what is done without him.
Glenn Fabry told me he didn’t waste time erasing or redrawing: his just did the right line all the time, it’s less time consuming! 
He explained that it was a matter of concentration and self confidence. Whenever he was drawing, he was thinking “I am the best in the world”.
Quino told me: You are very good! publishers are crazy if they don’t publish this!
It didn’t really improve my art, but when someone you admire as the best in his craft tells you this, it changes your own vision of your work forever, and your approach to publishers. A compliment is not very expensive, and can boost a career. Quino was good not only in his art.
When I was on Robert Rodriguez’ reality show Rebel Without A Crew, he told us two things that really changed my view on how to get shit done.
First, he told us the advice Steven Spielberg gave him when he asked about filmmaking on a bigger budget. “Don’t Blink”. The idea is that if you blink, or you’re not completely paying attention to something, you’ll miss important details. For me personally, this resonated as similar to the idea on focusing on the details, but paying attention to everything happening.
He also told me to meditate in whatever way I want. It’s not a matter of trying to completely close your mind or push everything out like you sometimes are told. For me, meditation is taking time in my day to breath, sit back, and let any thought or ideas that come to me just naturally come. I have WAY too many ideas burning through my brain. There’s no way I can get them all out and find nirvana LOL Instead, the key has been just letting the best ones take over and focus my thoughts for awhile in a place where nothing new is going to come in for a few minutes 
Being told upfront that you need to learn the rules before you can, effectively, break them, is one of the biggest things that was said to me (and I’m glad it happened while I was younger… so I could break bad habits early.) I remember when I was maybe 17 or 18 on a community forum, talking about how I wasn’t going to focus on anatomy so much anymore & only worry about making my work stylized instead.
Another artist went out of her way to type out a few paragraphs to me explaining why this was such a bad idea, why fundamentals were so important to have situated in your brain first, etc. She was absolutely right and I’ve never forgotten the way she approached me about it. Sometimes you just need firm critique if you’re going to grow. In fact, I think you always should be open to critique, and constantly connecting with others who are willing to provide it (then provide it to others in return.)