Booleans, figure out how to make them work. It’s touchy and tricky, but not random.
The Sculpt Tools Addon adds great functionality for operations between selected objects, without the modifier hassle. Put half your model in a cube object, select both, hit Separate. Voila - Model split in two, defined by the cube-geometry.
The same gotchas of the basic Boolean algorithms apply, but trial-and-error becomes much more convenient.
Confirmed to work fine with 2.65a.
For just slicing, the basic Knife tool works fine. There’s instructions in the 3d-view menu how to activate different functions, like “cut through”. Then you can hit V with edge loops selected to separate and move them.
About blackno666’s point “Move the separated part so you can edit it.”… use a numerical value, and it’s easy to move back exactly in place. Easy to forget when shuffling parts around, but very worthwile.