• Find a mentor, who could honestly say that your work is crap, and suggest on how it can become better.
• Copy great designs, learn how they were made, and think on why they choose that design decision, and not any other. If you can get to the bottom line of things, you'll be best at solving real problems with your designs, and your skills and experience will increase exponentially.

Just my two cents I learned from the past years.

Design interfaces that are out of your comfort zone, and stretch you into interaction design. No weather apps, portfolio apps, e-commerce, etc. - New interaction patterns

• Open-ended functionality ("playground").
• Apps that are used in edge situations ("while running", "underwater", "while drunk", "while distracted", "to control a nuclear power plant")

A multi-user drawing app. A presentation editor for mobile. A puzzle creator. A pedestrian walking app. A music player for people with one broken hand. An (adrenalin- and panic-proof) first-aid app. Etc.

I had an art teacher answer this about painting with 'brush miles.' There will some great advice in this thread, but this will be part of the answer. Mouse miles. It is a process that happens in your mind, a maturity evolves and a style that is born.

You can follow the trends, make sure you have a lot of white space around elements, pick tried and true font pairs, grab a beautiful color scheme from a website, or better yet, a pretty photo and you will be creating capable design.

But, to become good, or possibly great, you need to buckle down and work.

There are those that are born naturals, in all arts, but they are rare.

It's hard to overcome stagnation like that. The best advice I can give you is to try to learn something new about your craft every single day. Read blog posts, subscribe to newsletters, follow industry leaders and newcomers alike—all of these things can teach you something new.

Aside from learning, I would suggest finding an app you really enjoy that you think could be improved, and write a short case study on how you might go about improving it. Design those improvements on your own and explain why you made the decisions that led to them. This doesn't need to be a large, unsolicited redesign of an existing app or website—in fact, I'd advise against going in that direction. On the contrary, what this practice will do is allow you to spot pressure points in existing designs, how to spot and harness nuance to design something great, and how to keep aesthetics within an app consistently wonderful. It may take a lot of these exercises for you to learn a lot, but each one teaches you a few new things about design and thinking creatively.

I'd also suggest experimenting a lot. Try new techniques, explore new media, and apply knowledge of your current craft to the exploration of a new craft. For example, I always thought of myself as a web designer up until about two years ago. Then I decided to experiment with branding, then with poster design, then with editorial design, and the list goes on. Now, because I experimented with new media, new areas of my craft, and new techniques, I have a much broader and more well-rounded body of work.

The last thing I'll leave you with is that I think it's important to design a lot of things. It doesn't matter what you design (as long as you're doing it for yourself and not a client). Nobody ever has to see these things. Some of them might be terrible, ugly, and ineffective—in fact, most of them probably will—but that's 100% okay. This will teach you about iterating, about researching, about trying new things and taking new risks. It'll teach you one important lesson above everything else: that failure is part of the process, and you need to keep overcoming it to create something better.

Try more, you must.

In all honesty, skills plateau sometimes. Expand your toolbox by doing random new things. Stay hungry, as you are now, and you will feed yourself with new skills and insights.

Also, do usability testing. Talking to users will help you make better choices in next projects.

I would start expanding your understanding of design through various courses such as Lynda.com or HackDesign and stretch yourself by taking on personal projects that go outside your comfort zone.

My advice is simply practice practice practice. The more you do something, (theoretically) the better you get at it.

• Ask questions and demand context. Ask why this matters and why we should work on it.
• Recognize what you understand and what you assume.
• Work with customers to rapidly prototype your design.

Find problems you're interested in. Create stuff to solve those problems. Rinse and repeat. After 10,000 hours you'll be a great designer.

Plateaus are going to happen no matter what. When I got started I did have a mentor who always seemed to produce amazing work. Luckily he shared some tips and tricks and even PSDs with me so I can deconstruct his approach. Looking back years later I realized that I have grown as a designer myself by becoming better and more efficient than ever before thanks to the drive to get better.

Simply put. To get better at something you have to practice. It won't happen overnight. Just keep at it and if you hit a road block try a different approach.