10-day schedule
I used to train everything over one week, but got stagnated, especially in chest and shoulder exercises. Now I rest more, and train 4-5 days per week, but split my routine over a 10-day period, and have found fast improvements. I train triceps, legs, and back twice during that 10-day period, one heavy day, and one light day of only two or three sets (ayy, the burdens of an ectomorph...). I treat abs and forearms like any other muscle group (the way to get cool abs is through diet and aerobic exercises, not overdoing abdominal exercises).
Training chest, legs and shoulders leaves me too wiped out to train anything else on the same day, even though I only do four exercises for each, so when I train those groups I train nothing else. Since I started training triceps and biceps together, I've noticed a considerable improvement in my arms.
The order is also important, bearing in mind that secondary muscle groups also get tired during training (i.e. shoulders and triceps while training chest, biceps and forearms while training back, etc.).
Day 1-chest
Day 2-biceps, triceps
Day 3-rest
Day 4-legs
Day 5-back, forearms
Day 6-rest
Day 7-shoulders
Day 8-light legs set, aerobic exercises, abs
Day 9-light back set, light triceps set
Day 10-rest