The tricks are to (a) make sure the ganache is chilled & firm, (b) make sure you use a parchment sling (see photo below) so that you can easily lift the brownies out of the pan for cutting, and (c) wipe your knife off between cuts. The parchment sling will allow you to lift the brownies 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 ganache is sticking to the knife) will prevent smearing and crumbs.