I have to confess that for years I ignored this advice, because it sounded so stupid. I wrote to some of my favorite artists and asked them if they had any advice, and they all said the same thing: PRACTICE. DRAW. DRAW ALL THE TIME.
I took offense to that because it seemed to me that certain people (ie the people who were giving this advice) had obvious natural drawing talent, and no amount of practice on its own was going to overcome my lack of talent. I countered that I've been drawing since I was a kid, and never really got any better.
There is some truth in the fact that some people have more natural talent. But even people with natural talent, it turns out, have usually developed that talent by practicing intensively. And that's the key: not just drawing sometimes and calling it practice, but focusing that practice and making it constructive.
So take the people with the gobs of natural talent, and just ignore them. They don't come into it. And they're a very small percentage of amazing artists. Most of them really have gotten good via practice. Many of them go to art school, but that's really just institutionalized practice. All the practice they're forced to do, that's the real reason people who go to art school end up better at art.
One artist explained to me how he improved. He said he was terrible at drawing faces. No matter how hard he tried, he just couldn't get it right. So he created a practice schedule. Every day, he would draw a face. In the beginning, most of them were awful. But that wasn't the point. It didn't matter that each one wasn't good. Very, very slowly, he started to get better at it. He started to learn from his mistakes. Not even consciously. It wasn't that he'd draw each day then go "aha, I seem to have misplaced this line, so tomorrow my work will be better." Day to day it didn't seem to make much difference. But at the end of a month, he was amazed at the progress he'd made. Like if you measure a little kid's height every day. They grow fast, but you don't notice the difference until the annual height measurement. And then, after he was so sick of drawing faces that he wanted to scream, he picked the next thing he was terrible at drawing, and did that for a month. Now he's a professional artist.
So I started doing this myself, and it has made an incredible difference. Pick the thing you think you're worst at drawing and just draw it every day. Use whatever cheats and tricks you need to. Use a grid. Trace. Copy other artists' drawings. Pick a single photo of a face and copy it every day for a week, then compare all the copies. Try to catch details of people's faces in real life and get them on paper. Fail miserably. Keep doing it anyway. EXPECT that your work will be garbage, and laugh about your worst failures. Seriously. If you expect each piece to be good, you're going to get frustrated. Do your practice not with the goal of creating a work of art, but with the goal of training your stupid, sluggish muscles to obey your will, and your lying eyes to see more clearly. Then one day you'll be doing this and someone will walk by and be like "holy shit you're amazing!" and you'll be like "oh please, this is just garbage," and you will have, at long last, earned yourself a reputation as one of those amazingly talented people who doesn't even appreciate how talented they are.