As noted in the previous post on ascending slurs, descending slurs, too, should be practiced with the right-hand-articulated note on the offbeat and the slurred note ON the beat. This may seem odd at first, but, if the right hand maintains a uniformly quiet attack and the left hand maintains a uniformly strong, rhythmic, forceful attack, then the separation of the two hands will be properly cultivated and the possibility of playing them EVENLY begins to emerge.
However, the similarity ends there. The execution of clear, articulate descending slurs requires the application of opposing motions in the left hand in a way that is unique. I encourage considerable exaggeration of this gesture in practice.
The finger responsible for the lower of the two notes not only holds the string firmly against the upper finger's pull downward, but actually pushes in an upward motion (towards the bass side of the fretboard) at the same time as the upper finger pulls down. Prior to the note's release, the string will describe a slight zig-zag as the two fingers exert opposing forces on it. The release should result in the lower finger's sudden movement up (a slight extension).
When practiced in this way, the resulting note is louder and generally clearer. The string can sound very sharp and twangy if it is done correctly (more so in the case of adjacent fingers). This is, of course, an exaggerated execution designed to build muscular control over this phenomenon of inter-finger opposing motion. Once mastered, descending slurs can be played with any musically desirable degree of clarity or volume, and not be forced quiet, hostage to a weak technique and consequent weak execution. I like the analogy of the ball player warming up by swinging with two bats so when he has to actually bat, the single bat will feel relatively lighter in his hands.
I understand, naturally, that in many musical circumstances, slurs are used specifically for the purpose of their uniquely wispy quality. Nothing is more expressive or compelling than the slurred resolution to an appoggiatura, played by gentle left hand slur. However, there are countless examples in the repertoire of passages which require crystal-clear, reasonably forceful slurs. These methods of approaching them in the practice room will help.