I have a fat notebook with lots of pages. I write an index in the back, with a colored "tab" on the edge of the page corresponding with that index item. Any time I make a note about that item - call it "blog post ideas" for example - I mark the edge of the page my note is on with the corresponding index item.

I don't think I'm doing a good job of explaining this, I'll see if I can find an example of this online and link it.

Edit: I did a quick google search and found an example that's similar to what I'm doing. This works well for me: http://lifehacker.com/mark-page-line-edges-to-organize-your-...