I have just discovered this (only just returned from Spain myself). seb1 is correct we have an apartment in Spain and run it as a bit of a business.
We have a company in Spain that looks after the apartment, cleans it, hands over keys to guests etc, the company is run by an English couple who have both English and Spanish staff (which is useful as the area of Spain we have our apartment in a very popular area for Spanish holiday makers). The company is registered with the Tourist Board which makes the rental of our property in Spain legit. This company is located in the Mar Menor/La Manga area of Spain www.holiday-in-la-manga.com/eng/index.html they have groups of similar properties within certain developments and have been able to place them with wholesale travel agents such as Late Rooms, Ryanair etc for extra promotion.
I do a lot of our own promotion, mostly through internet holiday advertising, Holiday Lettings, Holiday Rentals, Owners Direct etc, etc, it's worth having a look and picking one or two sites that appeal to you and advertising with them.
As far as tax goes, we have only had our apartment since April so haven't had to do anything yet but I understand that there is a form you have to get from a tobacconist (??? I'm not making it up, honest ???) however I didn't come across a tobacconist's shop during our last visit and think I will need to ask for help to find this form, apparently if you have a maximum of two properties in Spain that you rent out you pay a flat rate tax on the value of the property but you cannot off set any expenses against this. This is my understanding from reading the tax section in a magazine about Spain but it's not exactly clear to me either but I am hoping to find out for certain very soon.
Very wise to make sure you have a solicitor, I think this was one of our biggest (and stupidest) mistakes. We went out on a viewing trip with a local company and were assured that we were being looked after and they were providing legal representation for us and turns out we were just dumped and found ourselves out in Spain in April trying to arrange the completion of the deal ourselves (with two small children in tow of course), then found ourselves in the meeting at the notary sharing a solicitor with the builder , have since tried for some kind of resolution with the company who put us in this position without any result just for continuity this is their website here www.bravoscotland.com/ their service was a big let down and I would not recommend using them.
We have been booked more or less solid all summer but things have tailed off now and I suspect they won't pick up until the Spring, we also have already taken a few bookings for next summer and have had a fair few enquires too.
Hope something I've said is useful and if there is anything you want to ask just give me a shout.