The best technique I used was for 3 part inventions I played each voice separately for a few weeks before slowly putting 2 voices together then 3. While doing hands together practice I also did voices separate. Weird, but that worked out for me.
JL
Missed the "recent" revival of this thread, but I was going back to some Sinfonias I never was satisfied with, just as finger exercises, as well as good, bread-and-butter music to warm up, and possibly to have in memory to give an example to someone live.
I did the D major 3-part last week doing exactly that -- each line WITH the exact fingering you choose for the final product.
And then I nicked an idea (new to me) from now-gone Bernhard, of optimizing learning by very methodically dividing the piece into short, non-sequential sections, doing each for a few minutes, going back to the computer to write code or read CompSci or Mathematics theory, then just repeating over and over.
Honestly, I found use, and still do, in individual voice work, but the most important to me is the methodical working on short fragments from various moments in the piece, HT in the 3-part.
But the other technique I use for sight-reading fugues from the WTC sometimes is to go EXTREMELY slowly, with the fingering I chose -- way slower than even a rank beginner could play it from sight. The intense focus and discipline helps the piece (usually all voices together, or maybe drop one voice if I haven't decided on a fingering) stick in my mind. It's very tiring, but if nothing else, it's something to do to explore some unfamiliar music without any technical problems.