Here are The Traveling Professor's best tips for making the most of a day trip to Versailles from Paris.
Weekends are most crowded. Wednesday is least crowded unless it is a holiday of course.
Dining options at Versailles are limited. If the weather is good, pack a lunch and enjoy a picnic.
The best way to get there is to take the RER "C" line from the center of Paris to Versailles Rive Gauche station. Be careful, RER C lines go to different destinations, so to be safe, pick a train with the signage "VICK" on the front of the train. Trains run frequently, about every 15 minutes or so. You will need to purchase a special train ticket to get to Versailles. However, if you have a Navigo Decouverte transportation pass, you do not need to buy a special ticket. All RER trains from the Versailles Rive Gauche station return to the center of Paris. The train ride is about 40 minutes.
I strongly recommend having pre-purchased a Versailles ticket or having the Paris Museum Pass before traveling out to Versailles. Lines can be astonishingly long to purchase tickets.
Upon entering, pick up a free headset for a self-guided tour. It takes about 90 minutes to go through the chateaux.
I recommend visiting the gardens and the Petit Trianon, on the grounds of Versailles. It was the home of Marie Antoinette. It is about a 25 minute walk from the Palace, but a shuttle does run to/from the palace for a small fee.