By plane: The easiest way to get to Porto Rafael is to fly to Olbia Costa Smeralda Airport and then either rent a car or get a taxi to Porto Rafael. There are also bus services to take you to Palau. Easyjet flies directly from London to Olbia in the summer and Ryanair has flights to Alghero, but that is quite far away. Otherwise Alitalia and Meridiana have flight all year long from Italy.
By car: There are ferries all year long from Genoa, Civitavecchia and Marseille, which normally operate overnight. Here are further travel details:
http://www.gosardinia.com/transport.htm

Once in Porto Rafael you will be met by our housekeeper who will take you to the villa.