Staring at the tires doesn't help me with backing up any. Knowing where the rear of the trailer is, and the direction is is going helps me the most. Of course you have to make sure your tires aren't running over anything, but I'm more visualizing the rear of the trailer.
Always, always pull forward further than you think you think you need to before you start backing. Even if you need to pull up an extra 100 yards so you know you can initiate a gradual turn, do it. The earlier you start to cut the wheel, the less amount of cut you will need. Think about the angles of it.
Don't wait too long to get back straight either. I think that is the most common mistake I see. If you realize you are cut too hard and not going to make it, pull forward slightly to straighten out both truck and trailer, and then try again. You can often fix over correcting by simply pulling forward 10ft and straightening truck and trailer.
Some things I say you might disagree with, or not understand. I pull a 48ft gooseneck during the week, and a 39ft Montana on the weekend, so I'm quite comfortable with maneuvering both forward, and backwards. The only way to get better is to practice, and learn from your mistakes.