I have to agree with a lot of what has already been said. Model something every day, even if it’s only 15 minutes. Last June (2014) I did a 30 day sculpting challenge to do a sculpt in 15 to 30 minutes every day for 30 days. Some of my best sculpts to date have come out of that, and the daily practice really helped to hone my skills as well as my familiarity with using them.
You mentioned taking some CG Cookie courses. One thing that I finally took to heart with Kent Trammells’ Shader Forge tutorials that I have since adopted into just about every phase when I model is using reference. Always using reference. Whether you’re modeling, texturing, posing – it doesn’t matter. Using references will make your work LOADS better. In fact, even my modeling, once I started working from references, improved. That’s why I now have a mirror and a tape measure close by my desk. The mirror is for helping with posing and other things (when I get around to learning that, anyway), but the tape measure has already helped me model my entertainment center, a coffee table, a side table, a kitchen table, and a night stand FAR more accurately and with more visual interest than I ever did before I started using them.
If you don’t have a particular object on hand to measure, doing a search for photo references is the second best thing. For example, you could just model a basic glass vase from “memory”, or you can do a search and find some antique Roman glasses and find lots of variety that, while still simple to make, will make for a much more interesting glass. Or instead of a basic wine glass, you might find inspiration from depression era glass and do a swirl pattern wine glass. (Both are examples of things I’ve done the last year.)
Really, the only limitation is your imagination, and you can create your own style based on something else, like making a Cerebrus (aka 3 headed dog) – from a Chihuahua (a recent idea that I have yet to implement).
And like Fweeb says… by implementing daily practice, there’s no need to decide between the Katana and the music box. Do both!
One last thing… Don’t limit your learning to 3D. I’ve been watching some videos on drawing, and I’ve learned some things there that are helping with my 3D, and I don’t just mean for character reference. For example, based on a style of drawing called gestural drawing (a form of quick sketching with the intent of capturing the movement within a pose), I tried doing a few gestural sculpts, something that helped me to see how you can capture movement within a quick sculpt as well. Not to mention the idea of “throw aways”… doing a particular subject over and over to increase familiarity with the object as well as improve speed and technique.
There are lots of ways to improve little by little, and even quite a few to help make a big “leap” in your quality. There are just two rules to keep in mind: 1.) Take action every day. Don’t just talk about doing it, actually do it. You can’t get better sitting on the sidelines. And 2.) HAVE FUN! Whatever you’re doing, have fun with it. It doesn’t always have to be perfect to help you improve. (Take a look at my character WIP. It has tons of accuracy flaws, but since it’s my first attempt, I’m letting a lot of those slide for the sake of getting through the project and will work on as I complete more characters.)
Good luck, and I look forward to seeing your work!