make sure you are using a serrated knife to slice it.
also some folks use a wooden slicing tray with "side boards"
that have slots to slice thru...sort of like a jig that is used
to cut lumber or boards at a certain angle (if you know what i mean)

like this..http://www.amazon.com/Norpro-Bread-Slicer-Crumb-Catcher/dp/B00004UE6T
or just google "bread slicing"...