This is sometimes a hard truth, but best illustrated with, well, illustrations: Questionable Content and Mansion of E both started in 2003. Jeph Jacques has agonized in his blog about improving his art, and it shows, both in the art and the payoff. Robert Cook has improved, and also now uses color. But there haven't been the quantum leaps there've been in Jacques' work. And yet they both have been doing several-times-a-week strips since 2003.
There's a basic assumption here that you may not share: I'm assuming that everyone wants to improve, to better themselves, and to make a living from their creative projects. If writing to you is a hobby or a way to vent, then this is probably not the blog post for you and you should go read the Questionable Content archives instead.
Learning more about writing is necessary to improving your own; it's hard to recognize ways to improve if you don't recognize what's good. Writing classes help, as well as reading good fiction and classics and popular mainstream fiction. The trick with reading, though, is that you need to analyze why they work, why they have staying power, why they're popular. It's called critical reading, and it's what your incredibly boring ninth-grade English class tried to teach you.
When you've developed a sense of what's worked for other people, reread some of your own work. What good stuff are you already doing? How can you improve?
Critically assessing your own work will do you a lot more good than churning out more of the same.
When you've reached the limits of what you can do yourself, it's time to have someone else read it and tell you what you can improve on. Finding someone who knows what to look for and will be honest with you is important. Some sites, like Critique Circle, are dedicated to providing this. If you happen to know someone you trust to critique like that, all the better. The other alternative is an editor. Once you have feedback, it's crucial not to start defending yourself. If they ask questions, don't answer them to the person critiquing you, answer them in the text when you rewrite.
This process, the rewriting and getting feedback and rewriting again, this is the kind of practice that makes perfect.