Watch the Fareed's Trade Secret #10 online guitar lesson by Fareed Haque from Jazz Comping Survival Guide
Unless you have a MOOG guitar or a guitar synth, your guitar has little or no sustain. Even Nigel Tufnel's Les Paul has little sustain compared to, say, a violin with a bow. So guitarists, mandolinists and other plucked string players have gravitated to tremolo, playing one or many notes very quickly to create the illusion of sustain. Simply holding a chord and strumming it with thumb, gently, softly, can create the illusion of a sustaining string section. Flamenco and classical guitar players have a whole compendium of tremolo and strumming techniques. Check out Paco de Lucia, John Williams. You can also try arpeggiating a chord with 2, 3, 4 or even all 5 fingers.