Here are three:
Composition: interesting points of view, above, below, foreshortening, distortions, what is in the foreground and background. Consider what you do when you have your camera in hand, how you choose the contents, edges, and positioning of what enters the frame, how different lenses affect how much is included.Though it's not mentioned in Harold's article, Brian Selznick's The Marvels offers many examples of these three tips... often in a single image, like here:
Subject Matter: What is (are) your subject(s) doing? Who are they? Do they have unusual expressions or clothing? Are they in an unusual predicament? Do they evoke an emotion? Are they up close or far away?
Lighting: Where is your light source? Dramatic lighting can often catch the eye... a candle lighting a dark room, the late afternoon sun casting huge shadow
The full article is worth checking out. Okay, one more tip that was really good:
Illustrate and Write On,
Continuity of Subject: Can you illustrate the same subject from different points of view, in different situations, showing different feelings?