The tricks are to (a) make sure the frosting is chilled & firm, (b) make sure you use a parchment sling (see photo below) so that you can easily lift the bars out of the pan for cutting, and (c) wipe your knife off between cuts. The parchment sling will allow you to lift the bars out and use a large knife and cut across without the pan getting in the way. Wiping the knife off on a warm, wet cloth in between each cut (or whenever the frosting is sticking to the knife) will prevent smearing and crumbs. You can also use the parchment sling overhang to help press down the edges to avoid crispy pieces.