Shoe selection

Mark Rippetoe or Mike Burgener (probably both) summed it up best in regard to shoe selection. Wearing running shoes while doing serious weight training is like "Trying to overhead press while standing on a mattress." Your shoes have cushioning agents designed to absorb shock, but not designed to stabilize you in a static position. If you do one legged squats you will notice a profound difference between doing them in your running shoes, and doing them barefoot or in a pair of Chuck Taylor's or boots. I had read about this before, but didn't realize just how much of a difference it made. I first picked up on it while doing one legged squats or kettlebell training in my military issue boots.
Part of the reason for this is that you actually have a sort of sensory mechanism on the sole of your foot that dictates how your body balances, and how it utilizes strength. To a certain extent the cushioning agents in your running shoes render this sensory mechanism null.
From the moment you put on your Chuck Taylor converse shoes you will feel a difference. Then move to some one legged squats, some kettlebell drills, or even barbell deadlifting, squatting, or pressing, and it will become even more apparent.
Often we will look at something like this and think it is a small thing, and not that big a deal, but there are so many small things like this that eventually they have a great impact on your training.