Is there a way to keep chilled cookie dough from flattening while cutting it? Whether using a serrated or straight knife, this seems to be a problem. I end up reshaping them by hand on the cookie sheet so they are round instead of oval.

Is there a way to slice the dough to prevent this flattening?   

PS: I should clarify that the chilled dough is in a roll shape.
Use a length of plain dental floss instead of a knife to slice: Wrap the floss around the roll, cross the ends of the floss over each other, and then pull the ends down and out to the sides to slice through the log.