I’m a DBA, not a programmer, but I subscribe to Jeff Atwood’s excellent Coding Horror blog because it’s well-written, funny and teaches good lessons.  In his latest post, he talks about database normalization – when to do it and when to avoid it.

Pay particular attention to the links to HighScalability.com – they have great stories about scalability problems and lessons-learned from really big sites like YouTube and Twitter.  Read those stories and know ’em well, because that’s the easiest way to learn some really expensive lessons.