After you have a guitar is a bit late, but here's what I have found over the years.
Anything that makes a guitar less solid, less stiff, tends to reduce sustain. The worst offender in this regard is tremolo setups. In my (heavily biased) opinion, most whammy-bar rigs don't have sustain worthy of the name.
A through-neck is more solid than a set-neck, which is more solid than a bolt-on. I haven't owned a bolt-on since I sold my '62 re-issue Strat, and I am unlikely to buy another one.
Since you already have your guitar, you are left with two options: sell or trade it to get something better, or modify the one you have. Either way, you have to part with some coin.
If the neck is bolt-on, make sure the bolts are snug. Don't get carried away, doing chin-ups on the wrenches. If you over-tighten and strip the threads, you have a problem worse than poor sustain. :eek:
pstring is right about the nut material. I like brass, bone or Corian. If it is set up properly, you shouldn't need a graphite nut to keep your axe in tune. A locking nut might seem like a strange addition to a non-tremolo guitar, but it would improve the sustain.
Adding mass to the guitar can help too. I've seen guitars with a thick brass plate glued to the back of the headstock. And the Yamaha SBG 2000 & 3000 series guitars have the bridge mounted on brass block set into the body. I replaced the tremolo unit on that '62 re-issue Strat with a brass block, and the improvement was amazing!
Stiffen the bridge. If you don't need a whammy bar, [u]toss it[/u]. Even a tunamatic type bridge can reduce the sustain if it is free to slop around on its posts. I modify mine to allow a lock screw on each side to hold the bridge down firmly once it has been set up.
Use medium or heavy guage strings. The .009" string sets a lot of players use don't have enough mass to continue vibrating against the magnetic pull of the pickups. You get better tone from beefier strings as an extra bonus.
As a last resort, get used to cranking the gain on your amp. Unplugged, a whammy-bar, bolt-on guitar strung with .009" 'spider webs' has about as much sustain as the average banjo, but with heaps of gain, distortion, and controlled feedback, you can get by. :D
Just don't expect to play jazz or blues with it. :rolleyes:
