refactoring database

sometimes I've got a situation while I have to change something in a database. This db is rather old and badly made, so I have to do something with it.
it's a huge problem, because redesining this database can't be done - this is working db.
i've found interesting site Agile DBA. you can found there many tips.
i've also found out interesting book about this
Refactoring Databases - Evolutionary Database Design.