Remember that the basic conflicts in fiction as well as life are man vs. man; man vs. nature; and man vs. self. The more of each that you bring into your plot, the more vehicles you have for building plot as well as suspense.

Your protagonist has to work for what he wants and the harder the struggle, the stronger your reader’s engagement will be. In most plots, there will be an overlap of at least two. Be sure to recognize them for what they are.