Unless you have specialized needs like making constant changes to your data, the most effective way to backup is to an external hard drive.
From what I understand, raid becomes usefull in critical applications where you write lots of new data on an ongoing basis and/or need high availability of data. Something like a hospital where a disc crash could result in lost x-rays or scans, or a database application where you need to verify data as something moves through multiple processes.
What type, and how often you backup depends on how much your data changes. If it's music you rip once, you really don't need to back it up every week.
At work we do daily incremental bacups (stuff thats new each day) and weekly full backups (everything on a drive).
I havn't set myself any formal backup schedule for full backups, but once every 6 months might be a good start.
