Deliberate practice is key to developing mastery of a skill. If you want to practice your programming skills, there are a wealth of resources like code kata.
What are the equivalents for test design?
Edited to add a bit more detail about deliberate practice:
In order for deliberate practice to be effective, it needs to be demanding, targeted at improving specific areas of your performance (and you have to approach your practice with specific goals in mind), continuous feedback on performance, self-reflection, and repetition of that practice over a long period of time. Code kata work because you have a specific goal, you can see when you've reached it (feedback), and compare your performance with other people's attempts (again feedback, and self-reflection), there are a lot of kata targeted at different areas (targeted at specific weaknesses, repeated practice).