Some Star Wars info cribbed from Slashdot:
Lucas wrote ANH on his own, but Empire (usually regarded as the best of the series) was primarily written by Leigh Brackett and Lawrence Kasdan. Lucas only "wrote" a small story treatment (that was several dozen pages long), which was dramatically changed and fleshed out by the real writers. The only reason GL even got credit was because it was "his" film, so he wrote the credits. His contribution was similar to James Cameron's contribution to the recent Spider Man film (all that remained from JC's treatment was the organic webshooters, incidentally).
Jedi, which uncoincidentally is usually rated between Empire and ANH, was done similarly although with only Lawrence Kasdan. Lucas basically phoned in a concept, and Kasdan pulled it off. Most importantly, it was Lucas who insisted on the introduction of the god-awful Ewoks [as opposed to Kasdan who wanted to have Kashyyk, the homeworld of the wookies - Lucas didn't think they were "cute" enough, given our experience with Chewbacca].
Unsurprisingly, the "new" trilogy is written solely by Lucas. He no longer is tempered by good writing, and so we have the largely panned films "The Phantom Menace" and "Attack of the Clones".