I think one of the biggest errors I see at the gym are people NOT pushing 110%. No gains will be made unless you push yourself to the max. The way I judge a successful workout is how hard it is to lift my arms up to shampoo my hair in the shower afterward, how much my legs feel like jelly walking down stairs after, etc. I also love feeling sore the next day. It equals muscle fiber recruitment. Diet also has a big impact, but... I won't delve into that here.
I'm also starting to add in a bit of cardio post-lifting so I can get below 10% body fat (post lifting is the best time to run as your body is in the optimum state to burn fat), and just bought a pair of vibram fivefingers in preparation. Excited! Research barefoot running if you haven't heard of it, it is enlightening.