if you decide to go, we followed Rick's advice about going PAST the main entrance and going "backwards" through the cemetery back to the main gate. It worked just fine. Make sure you use the map that is in his Paris guidebook so you see the people you want to see. We saw Oscar Wilde and Edith Piaf and Jim Morrison (who is not easy to find, keep looking).
When you're done and you exit after using the free restrooms, either walk straight and look for somewhere to eat, or take a right and walk along the esplanade back to the Pere Lachaise metro station.