Many times having many strong magnets in a pickup will dampen vibration especially when doing dive-bombs with your tremolo. If your strings become real slack they can stick to your pickup and really reduce sustain. Loose or vibrating bridges or saddles that can move or vibrate when you play and loose bridge components can keep the vibration from having a solid foundation between the string and body. Having a greater distance between the string and pickup will reduce the amount of magnet field from the pickup to the strings and can reduce sustain in a pickup. Using lighter gauge strings or having a pickup with less turns and DC resistance can reduce sustain in a pickup. The type amplification is important to the sustain and what you hear when performing. At times you get a nice response between the guitar and amplifier.