one thing that really makes a guitar have good sustain is how the neck is connected to the body. a set neck guitar (LP, SG, etc) has significantly better sustain than a bolt-on neck guitar (strat) because the neck and body are glued together- the vibrations transfer throughout the entire guitar much better.
a neck-through guitar has significantly better sustain than the set neck.

theres other things that help it out too- such as string tension. you can get a tighter string tension with an angled head stock. a hard tail as bluesman said also helps. a string-through has even better.

and the wood the guitar is made of can also contribute to the sustain. I dont know so much about that, but mahogany is good.

hope that helps, jiujitsu. :D