Improving your knitting skills can take decades. Going from a good understanding of the basics to real mastery takes time. That's OK since the process is and enjoyable one! there are so many differen ways a knitter can get help and continue evolving. Here are a few examples.
You can spend years working your way through a single pattern book, recreating each stitch and learning every nuance of how that person knits. Try switching and swapping new designers and patterns.
Try knitting an afghan maed of several different squares designed by different people. the project is portable, and you will find that there is more than one way to increase a stitch.
Go to a local yarn shop and take a class. Nothing too difficult, but you want to be challenged. You will have fun, hang around other crafting enthusiests and you will be exposed to new tools, yarns and techniques.
Find someone, anyone, who's a better knitter than you are, and talks the way you think. Perhaps ask about a specific problem. If the first person doesn't work out, keep searching, and give that friendship time to grow. If you are fortunate enough to have a friend that is willing to be your mentor, great! Take regular field trips to craft stores as well as yarn stores and compare the yarns and tools that are available at each.
There are literally dozens of books that are written. A good one to start with might be the Readers's Digest Knitting Handbook. Make swatches of stitch patterns you think are too difficult for you. If you don't succeed, that's OK, try it again in a few months. You will eventually be able to do that stitch pattern.
This is a bit of a repeat of earlier in the post, but it bares repeating. More heads help you to think about your project differently. If you are having trouble with a section, talk about it. Take a class, join a guild. Hook up with others in your neighborhood or community, whether you live in a small town or big city. Talk, fellowship, but most of all, have fun!