My favorite secret is to line the baking pan or dish with parchment paper. This allows you to cleanly lift the baked bars from the pan for slicing. Here are a few more tips for cutting those tasty bars. Make sure that the bars have fully cooled and set. Use a large sturdy knife or rocking slicer to make one large clean cut rather than a bunch of small ones.  Start by making a cut both horizontally and vertically in the center of the uncut bars. Then repeat this process in the four sections that you have made. The results should be 16 evenly cut bars.