If you want to put on weight then try this brand new idea......TRAIN YOUR LEGS! You said you want to focus on you chest, shoulders, and arms...well that tells me, that is what you are most likely doing now.....you would be surprized what a proper diet and training your lower body will do. Don't get me wrong, I hate training legs...but you have to do it if you want to gain weight! Trust me on this...several of my clients came to me in your same situation..."I can't gain weight" put them in the squat rack and WHAM....weight gain!