ehm...I hope you know about IK targets, autoIK(in 2.48a you can disable better, better work-together with autoik, some other rigging fixes) , floor constraint, track to constraints, etc, etc.
basically quite alot can be done with those. Without that, floating and sliding feet are easy to happen, angeneral posing is harder and more destructive.
the box trick is good, but i tend to prefer a floor, if textured, better (allows better idea of the move, etc, similar reason of the cube) That is, a huge, box at floor level.You can make certain bones never go bellow that floor

Tho is not magical. You need a lot of tweaking and often floor constraint is better not to use...
ISO time, x, y,z interpolation curves absolutely key to do animation well.