How-to Cut Baking Chocolate bar clean for perfect squares
You can score and snap.  You make a shallow score in the chocolate using a knife or other sharp object (gentle!).  Then you snap it.  It works better on harder chocolates.  Since all I have around the house this instant is a leftover halloween candy, I'll show the sequence here, but with this soft chocolate it would be easier to actually cut it through...

1) Score.



2) Pinch it. Here I pinched it between a plate and a cutting board.  You may be better off using a cutting board and your fingers -- so long as your hold it down stiffly with your fingers.



3) Strike it with a blunt object.  Or depending on the chocolate you can often just bend it with your fingers until it breaks.



(This basic approach is used in industry to make straight cuts in glass, tile, and silicon chips!  The harder the object, the better it works.  You can also try this on brittle candy, etc.)