Database desing for complex websites

[eluser]txomin[/eluser]
Not a CI question although all coding is being done with CI.

I have experience with SQL and can generally make sense of smallish database design. I am now at a stage where I want to move on to design larger, more complex websites. How do you scale properly? So far, though, I have failed to find adequate resources on the net (tutorials, samples, etc). My fault, no doubt.

Good folk, could you please point me in the right direction? Any leads appreciated.