Some people appear to be born with muscle*. I know lots of people who don't work out at all, have an office job where they sit around all day, and still they have a muscular body with strong (looking) arms, big chest etc. Me, I'm naturally slim, and however hard I work out, I seem unable to make a big change in my physical appearance. Yes, I do get stronger and more fit, but I look (almost) the same after a year of intense training: thin.
What can I do?
Several years ago I did dumbell exercises from Bill Pearl's "Getting Stronger" and supplemented my diet with a meat based protein shake. After a year this made me look more toned, but the effect was minor. I stopped this routine when doing lunges with weights caused knee pain.
At the moment I follow Mark Verstegen's "Core Performance" (the book), but have only been at it for a few months. I did not change my diet again except eat more soy products and nuts. The time is too short for any visible effects, of course. I'll very likely stop this routine also, because I'm experiencing shoulder pain from raining weights above shoulder height.
I don't mean to imply that these routines don't work for me, I just describe what I did to give you an idea of the status quo - and of the fact that I easily self-hurt myself, if I work with weights. I don't want to go to a gym (for reasons that are irrelevant here but definite), so I cannot work out using machines.
*Obviously people are not literally "born with muscle". There is a genetic influence and, as has been found, the muscles that children use around 5 to 6 years of age develop in the adult individual. Both influences cannot be changed by a post-adolescent adult, so "born with muscle" is a good short description of the situation.