Interesting you should ask that because I was just about to post about something similar. What you want and must get if you want to avoid bad tenants is Aval and an express eviction document. This will protect you from the kind of riff raft you don't want in a tenant.
I just found out an Aval is a rent guarantee from the bank. The tenant deposits between 3 and 6 months rent in a special account and the bank guarantees the rent for the duration of the lease. This means only those with stellar credit ratings can get one.
An express eviction guarantee (Maria do you know the Spanish name) is just what it says, pay 52€ and if they stop paying the rent you can evict them in 30 days or less.
In this economic climate it's a really good idea to get an aval it protects you.
This message was last edited by Rob in Madrid on 2/20/2009.