I'm certainly no expert and I do the same thing - fingers drifting in some cases or just being bad about judging how far to stretch for certain keys. I've particularly noticed this as I'm testing a new keyboard (TextBlade from
waytools.com ). It is so much more compact that, while maintaining horizontal distance to a normal keyboard, you don't have to move nearly as far vertically. Old habits die hard.
This means, at first, I was missing keys constantly. But I adjusted to the point where I could sometimes get a fair number of letters right in a row though other times I was a mess. But I kept practicing. One particular thing I did was, once I got some basic consistency, was to notice what letters I was getting wrong. For example, I use a dvorak layout. That puts "I" where the G key is and K where the V key is and X where the B key is.
First thing I noticed was that I was often hitting X instead of K on dvorak layout. The K required that I pull the index finger down and to the left more than my hands naturally wanted to do with the TextBlade's ergonomic structure.
So, I created various sentences which focused on the letter "K" in a text file and I would periodically practice typing and retyping those sentences. Maybe something like this: Lick the lucky sock like a knife.
I know, the sentence makes no sense, but I was just putting in words I was missing and doing the best I could to make it into a readable sentence.
And I got better. But then I noticed something else. Most of my errors with K started happening if the K had an "I" right before it. Such as the word, "like". The reason became obvious. The "I" required my index finger to stretch a bit to the right followed by the need to pull it back more to the left for the K. This emphasized my problem. So I did new sentences that included words such as, "Like a likable child, he liked to mike the bike".
It let me really focus on a specific problem. Hasn't gone away, but it is much better. I've regularly gotten over 60 wpm and even over 70 with 97% or better accuracy. But I still get some bad streaks sometimes.