One of the best ways is to be aware of your reaction to ads as you look through a magazine or newspaper. Apply that disinterested approach in evaluating your own ad. Use startling pictures. Look for a picture that is odd or amazing. A man going to work without pants, a close-up of a fly's head, an aerial photo of the acres of trees knocked down by the Mt. St. Helens' volcano are examples.
People love to look at other people.
The people in your photo should be doing something interesting or puzzling or fun, rather than just standing around like they're waiting for a bus. General crowd shots without a clear focus don't work well, nor do the usual shots of a few people standing at attention and wearing a plastic smile. It is hard to avoid stopping to look at a close-up photo of a person's face staring out at you. If the person is also doing something odd or startling, such a staring at you with two pencils sticking up his nose, all the better. Of course, you must have a reason for the photo you choose.