Probably best to start at the top and work down. In your hierarchy file do you have the parenting correct? If its not one result will be that the mesh splits when posing. Next have you got the joint rotations correct for each body part? The first order of rotation must be axial to the body part. The other two are less important but a good rule of thumb is to put the most often bent last. Finally are you using spherical fall off zones at the arm/chest and leg/hip joints? You'll almost certainly need them.