Some people I know index the heck out of all of their references using sticky tabs. I personally find that overkill since things tend to be well indexed in most books.
During my studying from any bound book or code, I find sections that have tables, equations, ect. which I have to refer to often in sample problems and I put sticky tabs on those such as "beam formula", "stopping sight distance", "pumping energy", ect.
Then there are the "loose references" these are sheets you find elsewhere like on the internet which explain a certain concept more clearly or in depth. These are usually just a page or two. I will take the full collection of all these loose references and stick them in a 3 ring binder, to make them acceptable for the exam. I usually don't need them, but it is comforting to have them there.