People exercise to stay healthy. The same idea applies to design. To stay sharp and fresh, you'll need to exercise your design skills. How do we exercise?
There's a cool website called Daily UI to polish those visual design skills. A prompt is emailed to you each day for 100 days and you can interpret it however you'd like. You set the constraints and the sky's the limit. Do keep in mind that these UI prompts tend to focus on single screens, and not so much on the overall user experience.
Get creative and flex those visual muscles!