I've always felt that in order for slurs to speak clearly, they need to be practiced with clarity: both articulately and fairly loudly. It is easy to perform them quietly, especially if the technique is well-formed--certainly in many cases, it is the delicate character of the slurred note that makes it appealing and expressive in context. On the other hand, in many cases its desirable to play the slurs forcefully, creating the effect of evenness with right-hand-articulated notes. In working on this skill, the following method helps:
Play simple ascending slurs, one slur per string, in order 6th-1st strings, and back down to 6th. (I like the formula spelled out in Carlevaro's Serie Didactica, Cuaderno No. 3.). Start with fingers 1-2, then 2-3, 3-4, 1-3, 2-4, and finally 1-4. Take each combination up and down the neck at least five frets to guarantee lots of repetitions.
Set the metronome to a slow tempo, say, eighth notes with qn = 60. Perform the RH notes on the offbeat. Slur to the beat. Use the pull of the sense of pulse to help teach the hands to play quietly on the beat (RH is piano) and loudly on the beat (slurs are forte). Practicing them this way will even them out.