Now that we have the thumb positioning covered, I'd like to discuss an element of playing that is not only applicaple to legato style playing but to everything and that is economy of motion. It does have it's roots based pretty firmly in legato technique for me though as with legato, the main task relies heavily on the abilities of the left hand.
I first came across this neat way of gauging left hand efficiency while I was immersed in studying the classical guitar. In fact, there are some extremely useful principles which I initially discovered during my classical studies which, when applied to the study of electric guitar, adapt exceptionally well.
Start by fingering the g note on the 3rd fret of the 6th string and play the note as you would normally. Take a mental note of how much pressure you are applying to the string to produce the note. Now release your finger from the fretboard all while making a mental note of how much pressure you applied to make the note sound cleanly. Now place your finger back on the same note but do not apply any pressure to the string yet (the finger should be touching the string but not applying any pressure). Now SLOWLY and steadily begin to apply pressure to the string in small increments. Keep doing this until you reach the EXACT point that you are able to produce a good clean note. I'd wager a bet that you are probably applying half the amount of pressure that you initially applied to get the note. Well, it doesn't take Einstein to work out that there is some serious wasted energy here! The general premise of this exercise is to be as efficient as we possibly can be so that we only use the EXACT amount of exertion needed at any given time. If this method is applied correctly, with practice, you will begin to develop a completely new command over your instrument.