I've read Rick's book on the subject, but still need advice on the best way to get there from Paris, and the best way to get around once I'm there. I'll be there in early October and will have only one Sunday to go Versailles.
RER Train, Line C, direction Versailles Rive Gauche Chateau. Most of the stations that go to Versailles run along the south bank of the Seine.
At the train station, buy a pass that covers the round trip train, Versailles and an audio guide. Once you get there, follow the crowd. It's a short walk if you are able. Be sure to hang on to your ticket.
Find out what times the fountains will be running and get out of the palace by then. All the fountains flowing is just beautiful. I can't imagine being there when they aren't. They add so much as you are walking around the gardens.
Be sure to walk all the way down the hill--don't just do the palace. We really enjoyed walking around the Queen's Hamlet. It's out of the Petit Trianon building, which is also covered with your pass. We enjoyed it more than the palace.
The pass Connie describes that covers both transportation and admission is the Forfait Loisirs. The RER C line divides on the west side of Paris. The electronic sign boards at the station will list the stops of each approaching train. Be sure to get on a train that is going to Versailles Rive Gauche.
Go early and try to avoid the huge crowds on Saturdays.
Friendly reminder: make sure you distinguish between the line to purchase tickets, and the line to enter the palace.
Thanks Tim for the name of the pass. Yes, there are two separate lines--one for those with no tickets and one for those who do. Just after you go in, you will see a desk for audio guides. Just show your pass and you will get your's free.