Two of the best pieces of advice I ever got considering art and drawings came from the very same person: A graphic designer who often stopped by at the comicshop. We spent a lot of time chatting about techniques and most of my knowledge about fibonacci and golden ratio I have because of him.
As for the points of advice:
One was that I should just take my time with my drawings and paintings. If I don't have a deadline, there is no need to finish it within a fixed timeframe. And I can work on it as long as I want, till it's finished. Or till I get bored with it. And never finish it and start something new. It's not like I'm bound in any way, right?
The second tip was to never shy back from starting from scratch. Whatever the reason might be. Just because I have just spent hours on that one detailed bit does not mean that the final image will look great if I build it around my initial hours of work. It can. But it does not have to.
So, I often I scrap drafts, paint over a painting or start from scratch. Just keeping everything because someone might like it and never risking anything, because it could ruin the existing parts? This is crippling for an artist and makes it very hard to try new things.
If you're scared to mess up:
I tend to mess up a lot. I just never upload the failures, which makes it seem like I don't make mistakes normally. But I do. I often have start again, copying the whole lineart AGAIN via light table (which is... boring as hell d: ), because coloring try number one looks horrible. Or because I realized halfway through the coloring that the focus is off. Or because I was simply not patient enough and now the whole thing looks horrible. Or because I noticed a very stupid mistake and can't really live with leaving it in the drawing and I have to redraw even the draft!
It happens. But each time it happens, I do learn something from it.
So, go wild. Risk a drawing. Risk a whole bunch of drawings! Never show them to anybody because you can. And if you show them to people, don't shy back from scrapping them anyway. Because this kind of courage gives you the chance to learn and be better artist than you were just yesterday.
A few hours of work are a small price to pay for progress, if you ask me.
No matter how good your drawing is right now, the next drawing can be even better.
And if you're not drawing yourself, keep this in mind when you try to convince an artist to not scrap a drawing.
(I'm especially looking at those people who charge the trash bin whenever they come to my place