| I found my degree of right-hand control improved appreciably when I practiced just playing steadily and as fast as was comfortable on one note, just droning away. I would see how long I could keep a drone on E going, then I would move to A, or fret a note. I would drone on that note as long as I could until my hand got tired, then rest with some left-hand work, then go back to the drone. Do this for 20-30 minutes a day and make sure you get every string in. It's muscle memory learning exactly where the string is and how to accurately hit it every time.
THen I started doing arrapegio chords across all four strings. I had a little riff I was doing with a b minor chord (7997) up to a d major (10 12 12 11), down to an F# (2442) and so on. After I did the above exercise for awhile, three or four days, I came back to this arrapegio thing and it was much cleaner.
As you are droning on that single note, vary your attack angle. See how many different ways there are to hit that string and make it respond differently. You can swipe it with the tip of the finger, or you can get the meat of the finger into it, and so on.
Also, I play with index and middle fingers. Try wiggling the tip of just one plucking finger back and forth to make your drone. It is a bit awkward at first but it improves articulation dramatically. Think of how players like Jamerson and Geddy Lee do very fast runs with just one finger. I tried that kind of thing with both fingers and while it isn't my technique (yet) it helps me out tremendously with my two-fingered technique when I only pluck in one direction. | 
