Step 1: Skeleton
Sketch your basic structure outline. For the torso we will of course need the vertebrae, ribcage and pelvis. Don't worry about sketching these the -exact- shape of the actual bones.

Step 2: Muscles
Sketch in the general shapes of the different sets of muscles. Check your muscle layout with an anatomy chart or constructive anatomy book.

Step 3: Skin
In this stage, you're making your sketch look like it is a normal, naked torso. Decide which areas should be defined, darkened, or lightened. Good references for this stage would be a fitness magazine, sketches from a life drawing class, or a constructive anatomy book.

For more information on skeletal and muscle structure, please consult your local bookstore. If you can't afford a decent book on anatomy, bring a sketchpad and a pencil with you.