whiz, I can only answer regarding Barcelona. Tickets usually go on sale to members from September 1st or thereabouts. Joe Public can buy from a couple of weeks before each match. The only sell-out game will be the one with RM, all others you can buy the week before, either on the web or at the ground. DON'T use the tout sites, you'll pay through the nose for poor seats that you can buy directly from the club.
Remember the day AND time of the match are not confirmed till about a week or 10 days before the match so you need to plan to be in Barca Saturday AND Sunday evening - not exactly a hardship.