To get better at something, you have to practice well.
"Those who developed deep expertise in a challenging domain did hours and hours and hours of Deliberate Practice."
-Kathy Sierra, Badass: Making Users Awesome
You have to practice that skill in a way that pushes you out of your comfort zone. If you are always practicing a rote skill that you've already mastered, then there is no progress to be made. It sounds obvious, to me at least, but I still find myself frequently in the trap of practicing poorly. As I tried to learn piano last year, I put in time everyday on scales and songs that I had already figured out. I was getting bored with those and also not getting any better at piano. It was frustrating. But it also felt like I was doing the right things -- putting in time and grinding away. As Sierra points out in her book,
"The wrong ways to practice make sense."
"Deliberate Practice is always just beyond our current ability/comfort zone."
Pushing yourself out of your comfort zone is balancing act though. You won't make any progress by attempting to practice something that is too complex. I also found myself in this trap. After I would finish cruising through my scales, I'd stumble my way through large portions of a piece that I didn't have the skill to play well. I should have been focusing on small pieces of it or even working on a slightly easier song.
Deliberate practice is when you work on a skill that requires 1 to 3 practice sessions to master. If it takes longer than that, then you are working on something that is too complex.
Find that sweet spot of a practice session putting you out of your comfort zone, but not being unachievable. If you keep stacking these bits of deliberate practice on one another, you will be incrementally stacking the bricks toward becoming an expert.
The other challenge of steady deliberate practice is having a progression path for that practice. In order to engage with a skill on the edge of your ability consistently, you need to have an idea of where you are headed and you need to have some sense of the incremental steps that will get you there.