Good point, distance and angle facing the amp makes a huge difference, however you may not always want that overdriven feedback sustain. Vibrato will also increase the level of natural sustain as the string is forced to continue resonating longer than simply fretting it... it does alter the sound though (by adding vibrato!), which is mostly a good thing... unless you want even sustain without vibrato. If that's the case then look at compression, good pickups (actives tend to offer longer sustain) and a gainy amp setting. Also amps and other analogue gear give far superior sustain when compared to modelers.