Of course the easiest way to advertise your house for sale in Spain is to give the details to one or more estate agents and let them do the donkeywork. If you're not living in the property you can simply give the agents spare keys and you won't even have to be bothered with viewing visits. But it's important to bear in mind that agents in Spain generally charge much higher commissions on sales than those in north European countries.
It's common for Spanish agents to charge 10% or more of the sales price and they put that fee on top of your asking price which obviously makes your property less attractive to potential purchasers (especially when you consider the buyer already has to fork out an average of 10% of the purchase price for various fees and taxes).
So if you have the time and inclination it's worth considering trying for a private sale without using an agent who will inevitably cream off a sizeable chunk of your ultimate profit. Browse the Internet for leading web sites that advertise Spanish properties for sale because this is an increasingly popular way for foreign buyers to search for their dream home in the sun. Some web sites let you place photos and details of your property for no charge but you generally find that these are the lesser known sites that are trying to build a presence on the Internet and therefore need as many properties as possible to improve their search engine ranking. A better bet is to go for an established web site that offers you a full page advert with colour photos and comprehensive details of your property for a set advertising fee. You pay a one-off fee to advertise for say six or 12 months and if you manage to sell the property in that time there's no commission to pay. If the property doesn't sell you get the option to continue advertising which is normally far cheaper than paying an agent's commission.
If you go down this road, bear in mind that it's up to you to paint as attractive a picture as possible of your property - the web master doesn't usually take responsibility for the presentation of your particular page. So make sure you supply the best possible photographs of you villa or apartment (even if you have to employ a professional photographer) and put together some lively and informative text that really sells both the property and the attractions of the surrounding area.
Alternatively, you could do what many owners in popular ex-pat areas do - put a "se vende" (for sale) sign with your phone number on the wall or gate of your house and hope to attract buyers who are tramping the streets trying to avoid agents' fees just like you! It can work to the mutual benefit of both parties and there's no risk involved as long as both sides use a good independent lawyer.