Best practices for Multibody parts / Top-down Assembly design

I'm guessing this scenario happens often, but I could not find any material covering the subject matter.

You're doing some industrial design (Realize shape for example) and the shape of the main body of the product is completed in 1 part file. What would be the best practice to:

1. Split this body into, for example, a top cover and a container
2. Convert the cover and the container into components and place into an assembly

OR, 

Is there any written material on such design approches with NX.