This is how I understood it from consular officials. (Again, I don't know if immigration control agents follow this policy or not.) If you enter Spain on Jan. 1, 2003, you automatically have a tourist visa good for 90 days, or roughly through the end of March. That is the only tourist visa you can get until July 1, 2003, when you may enter and receive another 90 day tourist visa. If you entered on July 1, 2003, you could not obtain another tourist visa until Jan. 1, 2004. Another way to look at it, every 6 months, you can go for 90 days. If you want to stay longer or go more often, you need to contact the Spanish consulate in your home country to apply for the appropriate visa.
Do a search of these boards. Lots of people just overstay their visas and keep a low profile. You have to decide for yourself. As long as you are not earning wages, you are probably a little better off as far as incurring wrath is concerned.