As with all forms of writing, the perennial advice for humor writers is “write what you know.” There's a wealth of humor in the world around you. Chances are, you're just not paying close enough attention to realize it. Tune in to your surroundings. You'll find enough material for several strips.

As a collector of humor, you will find you're able to do much better if you refine your collection process. Get into the habit of carrying a small notepad and a pen with you wherever you go. Even if you're completely confident in your ability to recall the gag later, make a note of it anyway. It doesn't have to be detailed, just enough to spur your recollection.

Be especially vigilant about keeping your notebook by your bedside. Your subconscious mind is incredibly powerful, often working out problems that have you puzzled during the day. If one of those puzzles happens to be the perfect gag for a cartoon you're working on, you'll want to be prepared to retain it.