Study other stuff! Design, at its root, is really just cognitive awareness and creative thought. Studying humanities, sciences, and other areas of the arts will boost your critical thinking, and give you a more rounded perspective on your work.

Design a lot of stuff, show it to people and get feedback, then refine.

Learn not to fall in love with your work, and realize from the outset that it is inherently flawed.

tl;dr - The more you do, the better you'll get.

presuming you're still working fulltime, and still learning domain-specific knowledge as part of that process, I would suggest:

• learn a related specific skill that will augment your craft. Life drawing, cartooning, logo design, writing, painting. Learn it pure, then relate it back to design (don't do a 'drawing for designers' course, learn drawing then see how it fits in your practice)
• learn a skill that doesn't have a direct relation to design and then see how you can relate it. Learn how to make a speech, or how to build a garden.
• volunteer for a community organisation where you can learn a whole bunch of skills and work on a bunch of projects. Find a community radio station and do their newsletter in exchange for learning how to host a radio show.
• volunteer for a hospice or support organisation and develop empathy and understanding (emotional skills are key to being a good designer).

I'd add to the above:

Try and learn first hand from others

Get to know your domain and customer / client expectations, use this as a measure for any "finished" design before you show it

Try to recognise when you fall into the "endowment bias" trap (eg. I spent five hours on this idea and I will defend it to the death in a review even though deep down I know it's not quite the right solution!) - not that I've ever done that

I'm in this situation exactly.

I'm copying a design I like every day. Just screenshot and tracing; replicating the things I think are awesome whilst working on projects that come up.

Just practice as much as possible.

I'm definitely looking forward to reading up on some answers here.

Personally, I would want to spend that time learning new skills, i.e. 3D modeling/animation.

Try to establish a core set of values and principles as a designer. I know, I know it sounds wishy-washy but doing so helps you remain true to whatever you do without giving into fads, trends, and dogma. Think long and hard about what that means for you.

This could be things like working for specific types of clients or companies, who share the same values as you. It could be working with only a handful of fonts or colours. These things aren't really related to technique (which you can improve by simply doing a lot of work as practice), but more so with what it means to be a designer for you. Hope that helps!

So much has been covered so i'll just add something that i found that helps personally. You can read all day and it's good to. But make sure you know when to step away and DO stuff.

Many times i'll get caught in a loop trying to stay so up to date and read way too much. When you do read though, try to talk to people about it. Conversations will provide better comprehension.

Keep your eyes and ears always open, I notice I get inspired more by the world around me than by looking for inspiration online... maybe the colors in a curtain, or the way a magazine is laid out. I think this might be because I get overwhelmed by the sheer amount of online info. By looking offline, I see less, but the attention I give to the individual things around me seems more meaningful.

• Practice.
• Find inspiration on your own rather than through others.
• Practice some more.

inspiration + application