Want more sustain? Reach for a compressor, which is like an invisible hand on your volume control. Compression amplifies soft signals to make them louder, and attenuates loud signal signals to make them softer. This gives a more consistent, sustained level.
The upper waveform shows a guitar string's decay. The lower waveform is the same signal, but compressed. The lower signal is louder, and takes longer to decay—but also note that it has the same peak level as the uncompressed signal.
But if compression attenuates peaks, wouldn't the sound seem softer? Normally, yes—but most compressors include a "make-up" gain control to bring the compressed peaks up to the same level as the original, uncompressed peaks, as shown in the above screen shot. (Note that the Firebird X compressor does this automatically.) And as you probably want to hear compression in action as well as see it.
Threshold sets the level above which signals get compressed. If the signal drops below the threshold, then the compressor leaves the signal alone until it exceeds the threshold again. Lower thresholds give more compression and more sustain.
Ratio affects signals above the threshold. A 1:1 ratio means that the output increases by the same amount as the input, so there's no compression. But a 2:1 ratio means that if the input increases by a certain amount, the output only increases by half that much. 10:1 means the output would increase by 1/10th as much, so that gives a lot of sustain—a big input change results in a pretty constant output.
Attack is the compressor's "reaction time" to input level changes. With a fast attack, as soon as the input exceeds the threshold, compression kicks in. A longer attack time "lets through" more of a signal's original attack—like pick noise—before the compression takes over.
Release sets how long it takes compression to go away after the input signal drops below the threshold.
You can't argue with the laws of physics. Compression can't make your guitar's strings vibrate any longer, resurrect dead strings, or fix guitars with poor natural sustain. All a compressor can do is increase apparent sustain—not manufacture it. Sorry!
Control interaction. With almost all compressors, the controls interact. For example, you might want to lower the threshold to add more sustain to lower-level signals, but then the sound becomes too compressed. So, go back and lower the ratio. That way you'll still compress low-level signals, but the sound will be more natural.
Instant lead guitar: just add compression. Adding compression to an uncompressed rhythm sound can often turn it into a lead sound because of the extra sustain, which also gives an apparent level increase.
Instant rhythm guitar, too! Suppose you have great compressed lead guitar preset. Turning down the guitar's volume control puts more of the signal below the compressor's threshold, so there's less compression, less sustain, less apparent level, and more of a rhythm-type sound.
Smoother distortion. Adding compression before distortion gives a smoother, rounder, more "singing" tone. Try it!