You need to make sure you are up at the ticket office by at least 10.30am on the day of the game as this is the only time tickets are on sale to the general public, the touts will tell you that its sold out but dont be fooled, we were able to buy excellent seats in the Grada baja just 12 rows from the pitch behind the goal for the "massive" price of just 35 Euro (23 quid) each, it will help if you can speak a bit of Spanish and know where you want to sit, if you go to the Spanish version of realmadrid.com there is a stadium plan with prices, hope this helps
Hala Madrid!