Electric1Gypsy
Inspired
True but it works today
To sustain a note you have to be at the right distance from the speaker.
When the sound coming from the speaker is in phase with the note you're playing, you'll get (better) sustain.
When they're out of phase, the note will drop dead.
Santana marked the positions on the stage for each note.
To get a natural sustain at low volumes, you might want to install a Sustainiac.
As a few people mentioned, this comes from stage volume and clean technique. You sometimes have to be muting the strings that are not playing and be bending and adding vibrato very cleanly without sounding any other strings.