If you want to be ectomorphic (long and thin):
Do more aerobic exercise, running and biking and ensure you work at a steady state (70 per cent max heart rate). Avoid explosive activity.
Use exercises that allow full range of movement - swimming, isokenetics or yoga and take more time to fully stretch each muscle.
Concentrate on eating complex carbohydrates such as pasta and potatoes and drink plenty of tepid water.
If you want to be mesomorphic (athletic and muscular):
Work on short bursts of explosive, dynamic and intense aerobic activity near to maximum effort - short sprints, jumping and punching.
Do more weight-bearing exercise - pump iron. Use heavy weights, up to 90 per cent of your maximum capability and complete at least three sets of each exercise with no more than seven repetitions per set. Ideally isolate one muscle group at a time and work it until exhausted.
You must have plenty of rest between each workout to allow the muscle to grow.
If you want to be endomorphic (rounded and stout):
Buy a video, some beer, order a takeaway, sit back and enjoy.