Does George Lucas have any plans for more Star Wars sequels or prequels? Proper movie sequels, not any supplementary animated shows or stories.
When this question was originally asked, it looked like George Lucas didn't have any plans to make any more Star Wars films. That hasn't changed, but we all now know that the franchise was sold to Disney, Episode VII has been released, and more Star Wars films and series are on the way.
Unfortunately it doesn't seem likely. Here's a quote from an interview he gave in 2008. (The article is Will Lucas Extend His 'Star Wars' Story Beyond 'Return of the Jedi'?)
"There really isn't any story to tell there. It's been covered in the books and video games and comic books, which are things I think are incredibly creative but that I don't really have anything to do with other than being the person who built the sandbox they're playing in." He continues, "I get asked all the time, 'What happens after "Return of the Jedi"?,' and there really is no answer for that. The movies were the story of Anakin Skywalker and Luke Skywalker, and when Luke saves the galaxy and redeems his father, that's where that story ends."
With the recently concluded (November 2012) acquisition of Lucasfilm by the Walt Disney Co., Disney is planning to release Episode VII of the Star Wars saga in 2015.
"Our long-term plan is to release a new 'Star Wars' feature film every two to three years," said Disney's Robert Iger.
George Lucas will not be directing the new films; he is retiring.
"It's now time for me to pass 'Star Wars' on to a new generation of filmmakers," he said.
EDIT: Plans for the new film have firmed up, with J. J. Abrams signed on as director and Abrams and Lawrence Kasdan to do the screenwriting. John Williams will return to score Episode VII. Shooting is expected to begin in the spring of 2014 and the film is scheduled to hit theaters 18 December 2015. Source starwars.com.
Ther's is still rumor that there will be a sequel trilogy. Here the last one I'm aware of :
[...] fans can expect the new trilogy after the entire saga is released in 3D which is expected to be complete around 2015 or 2016.
Technically, George Lucas doesn't plan on making any more films, as he sold the rights.
However, Disney has multiple Star Wars films planned, with set release dates.
The first, Star Wars: Episode VII: The Force Awakens came out December 18, 2015.
The next, Rogue One will come out December 16, 2016.
Many years ago, I got a chance to talk to Anthony Daniels at a Gen Con. I specifically asked him about the chance of episodes 7, 8, and 9, which Lucas originally had planned to make.
Daniels told me it wouldn't happen.