One of my takeaways from reading "The Talent Code" was to practice at the edge of one's abilities.
This may have become one of my top guiding principles for my own sessions. Something I always search for and question.
What does that exactly mean?
First, it helps to pick a spectrum and roughly understand where you are.
Then go practice around that spot, trying to push yourself further on that spectrum.
For example, that could mean serve faster, swing smoother, jump higher, spin spinnier, use something heavier, etc.
It doesn't have to be something super exotic. Skateboarding or even sliding with the "wrong" foot forward was plenty edgy for me for a long while.
Looking for your edge goes back to finding some new effective stimulus that'll - in a positive way - force you to try something new.
First, it'll be uncomfortable. (And as always, be mindful of and take precautions against injury.)
If that "uncomfortability" starts to go away, and the required effort decreases, you might be on the right track.
When it eventually feels easy, it's time for the edge.
While that "edge" mindset is probably *the* key to progress, be mindful of overuse and exhaustion. Both physical and mental.
After some practicing on the edge, it's also fun to call upon something else that you already had acquired and enjoy that for a while. (After all, that's what most people seem to do later in life...)
And/or look for an edge on another vector, that's maybe "orthogonal" (i.e. independent). So again something new, not getting bored, no avoidable RSI.
If you already ended up here, then look for that edge on the other side of your body as well, and bring them up to par :)