We used to advertise our apartment in Spain on Owners Direct and got many many applications for holidays in the high season. So much so, that I was hard pressed just to answer the emails. It is easy to rent out a holiday apartment in the high season but the difficulty is renting from September to June.
I have used DoneDeal successfully for specific periods of rental and I recommend it. We placed free ads in supermarket boards too with some success. It takes years to build up contacts and eventually we have successfully relied on word-of-mouth. Forget about national newspaper advertising; it is expensive and useless.