breaking off chocolate from a block of chocolate
One option is to make sure it's well wrapped (preferably loosely), place it on a soft surface, couch or bed, and bring your weight to bear on it - pressing with something like a knee will be easier than using your hands.  As the surface yields under your weight, it puts upward pressure increasing towards the edges of the block while your weight places a downward pressure in the center, and the chocolate will tend to snap between the forces.  

Using force like this, it usually breaks along the indented lines, not always precisely but pretty close - they are weak points in the chocolate's structure.  It works best with the pressure held at the middle of the chocolate piece, and it will also tend to snap in halves, so to further break down the chocolate you may be able to move the pressure to the middle of the new pieces.  There will come a point where the pieces are too small, there's not enough space within the wrappings to let it bend, or the pressure is to spread out to keep it snapping - but you can break it down into pretty manageable chunks like this.

I have a pound chocolate bar that's pretty thick, and quite sturdy - more so than I'd like to snap by hand - and this is the method I used to break it down to size pretty quickly.  I found it wouldn't easily break smaller than a two-square rectangle, but that's already much easier to work with.

If you want smaller pieces, or more control, you can take a knife (I used a cleaver) and use it to chisel the block apart.  An actual chisel would also work here.  You would set the blade exactly where you want it to break, and tap the back of the blade with something heavy, using increasing force until it splits.  This works with even pretty small pieces and with much more precision than other ways - for example you can use the smooth back of the chocolate block to position the knife, so the indentations won't interfere as much, to make half-squares, or long slivers or wedges, or clip off corners or whatever.

It is much more controllable to use multiple, almost gentle taps (using the weight of the hammer) to weaken it and let it crack apart rather than use force of muscle which can go awry.  I used the pestle from my large granite mortar &amp; pestle, but you could use a hammer, a rolling pin, or any other decently sturdy object. 

Last option would be to position the chocolate with just about the amount you want overhanging the edge of a hard surface (like a table or counter), and knock it off with a sharp blow.  It really works best if the amount you want is enough to hang over the edge by, at least, more than the thickness of the bar. The chocolate will fall from the table or counter as it breaks, so make sure it's wrapped (loosely enough to give it room to break or the wrapper will break with it) or else have something in place to catch it.  You can strike with a hammer, rolling pin, or other decently sturdy object.  This is less controlled, and it can end up with your chocolate on the floor, but it does work in a pinch.