Not sure what area you are in but I can only comment on the Costa del sol. This is probably the worst time of year to sell as most people are only interested in holidays and buyers are few and far between. However, I don’t know any great techniques but can offer some advice:
1) List your property with an agent who is part of a Mult Listing Service, such as the Interagency Network in Costa del Sol. This will mean that your property will also be available to anotther 200 or so agents for them to sell too. Commission can be up to 10% (ouch).
2) Make sure your apartment looks impeccable inside and out. Apply some “house doctoring” techniques if necassary. In an apartment it is particularly important to create a sense of space.
3) If you get desperate you could always try a Spanish property auction such as Auction property Spain, although you may not make so much of a profit.
Hope that helps.