Everyone should know the cheapest and best from of advertising is word of mouth. If somebody rents your apartment and has a fabulous time they may tell two people - guess where those two people will stay if they fancy a holiday in Calabria.
Conversely they will tell a lot more people if they have a bad exèrience, just because thats how human nature works. As Warren Buffet says "it takes years to build a reputation and seconds to destroy it."
Think of ways that will make your apartment stand head and shoulders above the competition.
The first step is how they pick up the keys, think back to previous holidays you have had where you you have worried about how easy everything is going to be when you arrive at your destination. Ensure this part of your tenants experience is seamless and unambiguous.
Leave some local wine/produce and a welcome basket for your guests upon their arrival, include easily understandable local information such as good restaurants, bars etc. Start a guest book, encouraging people to leave information good and bad about local services.
Make sure you are contactable for your renters!
Have a look at this website that is geared wholly towards property rental.