This is an example of using advertisements from magazines. Take your magazine and find things that you are instinctively attracted to visually. There should be no rhyme or reason with why you are attracted to something. It can be as obvious as a bright yellow daisy, or as random as a paragraph in a certain typeface. Cut it out and either glue it or tape it to the first page of your sketchbook. It does not need to be in any order, you can randomly put it on the page. Go with your first thought on where it should go. Repeat this step until you are done with filling up the whole page.
Go out into your environment and look at it as a purely visual thing. If you see something that inspires you, pick it up and save it for your book. This can be a gum wrapper or a real estate ad. It can be anything. Don't collect it if you are not attracted to it, only if it catches your eye. If you see something that catches your eye that you cannot pick up and put in your pocket, like a billboard or a wall's texture, or some graffiti on a sign, this is the time to whip out your digital camera. Take the shot and save it to print later on.
Here is an example of using photos mixed with more imagery from other publications. Print out all the pictures you took in your environment, cut them out. Put them with the things you picked up. Open your first page again and take a look at it. Then move on to the next page, and glue and tape your items the same way you did the first page--instinctively, without question, without reasoning. Fill as many pages as you have stuff.
Repeat all of these steps as much as you would like. You will end up with many books to look back on when you need an idea, or you need a visual reference that you can scan. These are like collages that are created from your subconscious because they are created through instinct instead of by logic and reason. It's a great tool to have when you are going through a creative block.