Agreed with @CrazyCatBird , I particularly like the workflow of either a) lasso or b) a temporary layer that I then erase parts of and merge down. I normally start using separate layers with alpha lock when the shape is difficult to paint precisely and it must be distinct from the other shapes near it. Like a silhouette of the character.
I don’t do that many paintings, and I’m pretty much a beginner too, but I would recommend doing studies with a focus on a parts that you want to get better at. If you see a nice detail in the reference, try to figure out how to get that effect.
Sorry for plugging my stuff here 
but here’s one study I did that I’m particularly happy with. I got a very nice sharp effect in a few places, like the bricks and the street pavement with a simple, hard edge and slightly contrasting lines. I used a slightly textured and not fully opaque brush, as that helped make the line a bit varied in intensity. My point is, just a bit of local contrast helps make things look sharp without that much effort! And I think the secret is to be very selective, if everything is sharp, then there’s not enough variety again.