I got a tip the other day to pratice my scales for a couple of days with my thumb not touching the neck. This teaches you just how little pressure is required to fret a note. This is only for practice though and should not be learnt as a technique as you would not have any control over you fretting hand at high speeds. It should help to force you not to cramp your hand when playing legato...
Try having your thumb in the middle of the neck when fretting the lower strings and your thumb about 3/4 up the neck when fretting the upper three strings... I hope this helps a bit. i am by no way a legato master but i have found that these techniques have increased my speed and agility on the eck by leaps and bounds...
Practice Practice Practice... Slow, Faster, Blisters...