Clanking the frets usually comes from aggressively slamming your fingers down too hard when you fret.
It seems like a kind of contradiction, but Real speed is a by-product of accuracy control, and a light touch.
To play fast and clean you really have to slow down and focus on teaching your muscles the proper motion.
This is an edited response form a thread about fret buzz, but I believe the exercise might apply here as well...
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This exercise isolates you left hand completely and helps build the muscle memory needed for clean fretting of notes.
It should be done in your usual practicing postion whether its sitting or standing (I always stand, since that's how I gig)
1.) Put all 4 fingers on the E string, 1 finger per fret, starting wherever that 4 fret stretch is comfortable.
press them all against the string just hard enough to fret a note
Here's a great
Gary Willis video on how hard to fret
2.)One at a time, starting with the pinky, lift a finger off the E and move it onto the A string.
keep the remaining fingers in place. Conciously move youre finger in three steps : UP, ACROSS, DOWN.
UP: should be quick and straight up off the string, no sideways motion to cause squeak.
ACCROSS: should be controlled, move in a low arc or parralell to the fingerboard, no flying out and back in
DOWN: Cocnetrate on landing firmly , with the minimum pressure, not slamming down.
3.)After you move your pinky, move the ring finger keeping all the other fingers in place
4.)Then move your middle finger, then your index finger keeping all the other fingers in place.
5.)after that, Starting with your pinky, reverse the movenment form A string to E string.
(If you are feeling ambitious, you can climb accross all the strings this way -its a great exercise.)
Do this slowly , concentrating on a clean three step motion: UP, ACROSS, DOWN.
It may even help to say it aloud to youself "up", "accross" ," Down" in time with your finger movement.
Keep your left hand relaxed with the minimum pressure needed to fret, move one finger at a time and keep the rest in place.
Once your left hand starts to get used to this motion, start plucking the notes you are fretting.
Chances are you'll get less buzz at this point, but if you still do, renew your concentration on UP, ACROSS, DOWN as you pluck.
Do not worry at all about speed:
the real key to clean technique is practing slowly and steadily until your muscles begin to remember the movements.
If playign a riff at speed causes your technique to slip, simply slow it down until your hands get used to the motion.
). They make a lot of difference. 
