For slicing rounds from a cylindrical log of dough, use a piece of dental floss (unflavored!!). Put it under the log and cross over the top and pull on the ends in opposite directions. This applies equal circumferential force and results in round slices! Also you can floss your teeth afterwards for improved dental hygiene =)

For cutout cookies, roll the dough on a piece of parchment, put the dough/parchment on a baking sheet, and chill. Cut out the shapes (still on the parchment/baking sheet) and remove the negative space (the "excess dough") so you don't have to transfer the cutout cookies to a baking sheet. Chilling again prior to baking helps maintain the shape of the cookie but most of the time I'm too lazy to do this.