Whichever muscle group you train, remember to train the opposing muscle group ie biceps/triceps.
Oh hell yes!! Something I have seen people do is totally forget about this when doing abdominal exercise. They do 50 million sit ups to get rock hard abs (which is useless if you have a layer of fat over the top anyway, but that's a different story) and then wonder why they have lower back problems. Imbalances in muscles cause problems. Or you see guys with huge guns and chests with chicken legs and no lats.
I like the "t-shirt body builder" analogy Squirrel
You could be the buffest, hugest bloke in the world and not be able to run 50 metres without collapsing in a heap or throw a ball across the room.