Vilanova - Sitges - Vilanova
As well as the regular train service between Sitges and Vilanova there is also an excellent bus service run by Plana. It runs via Sant Pere de Ribes (the nearest town inland and well worth a visit), and the local hospital of Sant Camils (which hopefully you'll never need during a stay at TdM)
Here are the current bus schedules...
First -- Mondays to Fridays
Sundays and holidays