Originally Posted by: tafs2) Use an eBow, to vibrate the strings. An eBow is a little effects gear you can pick up from most guitar shops. It is battery operated and produces an electro-magnetic field around the string you place it over, causing it to vibrate just as if you were using a violin bow (hence, e-BOW). These cost around $80, but are great tools to add to your arsenal of gear. They have 2 settings, regular and harmonics. In harmonics, the eBow emits an octave higher pitch, as if you were doing natural harmonics on the strings, and the sustain is, well...virtually infinite as longs as you hold the eBow on the string (and battery doesnt run out )

Exactly what I was going to post.

I'm sure there are some guitar synth options too.