Over at the AgileBits Blog (the makers of 1Password) Jeff has a nice discussion of picking secure passwords. His discussion is in the context of picking a master password for 1Password but it applies more generally. Much of the really interesting discussion happens in the comments so be sure to read them too.
He emphasizes avoiding predictability while generating long easy to remember passwords. I'm not sure I agree with everything he has to say but there are a lot of good ideas in the post. If you're looking for ways to generate some high security passwords, this post is a good resource. As I said, there is much wisdom in the comments too.