Excellent post, Agent. I am going to start incorporating some of these principles in my practice. I feel like like I haven't been practicing slowly enough and it shows in my lack of evenness in some phrases here and there.1) Identify your weakness
2) Practice your weaknesses, ignore your strengths.
3) Practice slowly the vast majority of the time.
So what I do everyday is after I warm up, I run through everything I'm working on at the moment. I make notes of what doesn't sound good that day. So if I'm running through my scales, I'll write down which scales are my weakest that day. Which ones I had flubs on, which didn't sound even, which didn't feel comfortable and made me feel anxious.
(There are some things I practice pretty regularly, but there are other things I never practice because they never show signs of needing it. If it ain't broke, don't fix it.)
Then I take the 4 or 5 worst, take out my metronome and set it at 60, and play them in 8th notes. (Actually, I put it at 60 but have the metronome beat out 8th notes with an accent on the downbeat so it's almost like playing them in quarters at 120.)
Then I play each scale 20-30 times. My goal is to get everything rhythmically perfect, eliminate all flubs, and get my hands/fingers feeling as relaxed as they possibly can get. I also make sure that the last 5 times I play something, it's dead on perfect. I call this the "5 time rule". If I make a mistake somewhere, I have to start over until I can play it 5 times perfectly.
Eventually, you reach a point where it feels like you're playing automatically, with little input from your conscious mind. This indicates that you're shifting responsibility for your muscle movements from your motor strip in your brain to the cerebellum. This is the ultimate goal for technique development and ideally how you'd with to play all the time. It's slow and tedious but more effective than anything I've tried.
I do the same for any technical exercise. For performance pieces, I run through them and make note of the rough spots and then practice those spots in the same manner.
You do have to play up tempo a little. Part of playing these things at tempo is knowing what they sound like at tempo. So you have to remind the ear by playing things fast. But most of your practicing should be done at a slow tempo. Somewhere around 60 BPM or half speed of whatever you're working on. That's where refinement of technique takes place.
So again, only practice what you can't do and when you practice those things, practice them many times at a slow tempo, trying to make them as perfect as possible.