Every public place in Spain - be it a shop, restaurant, bank, information centre or even bus - has a sign on the wall that says "This establishment has a complaints book for any customers that require them" (in both English and Spanish, although occasionally the English is dropped in favor of Catalan or Basque). It is a legal requirement for them to have this and it carries a lot of weight in Spain.
So, if you receive bad service in a shop or bar, ask the assistant or barman for the complaints book (point at the sign on the wall if you don't speak Spanish). It might be better to ask a different assistant to the one you have a problem with.
The forms are bilingual, so you shouldn't have a problem filling it out. Keep the two copies indicated (usually the green and white ones) and hand in the other (usually pink).
Each form is followed up and the establishment can suffer heavily if the law sides with you - if two people make a similar complaint they are even more likely to find in your favor.
You may be wondering how this will help you when you are only staying a few weeks in the country. Well, so seriously are these forms viewed, you often don't even need to fill one out - as soon as you ask for the book the service often changes instantaneously and you find you get what you want without having to go through with your complaint.
These complaint forms are a very powerful weapon for the consumer - don't be afraid to use them.
