1. Use templates for consistency in size and shape.
2. Use support for moving slabs -- I guess this is even more important when the slabs are softer. Use anything at your disposal that is close to the shape or the size of what you are making.
3. Pre-decorate the slabs when they are still flat as much as possible and minimize fussing once they slab is "assembled" into their final shapes. Give your work time to firm up before you have to handle it again.