When working the back, it's important that you keep your back arched inward (toward your stomach) and your stomach pulled back toward your spine.
The organs in our abdomen form a "fluid ball" when we apply pressure to it. We say this because that region is comprised almost completely of water. Since water doesn't compress easily, this region is fairly solid and can support a great deal of weight.
The form I mention above is critical to compacting this fluid ball and thus creating a strong support structure upon which to support your back bones. Some people wear weight belts to reinforce this fluid ball (but it's not recommended you do this regularly because it weakens your ability to do this naturally, or in real-life situations).
Finally, this is the most important one:
DON'T TEACH YOURSELF DEAD LIFTS!
Dead lifts are an extremely dangerous maneuver (along with the power clean and jerk) that you should always learn from a qualified trainer. Since heavy weight is pretty much a requisite for this exercise, you run the risk of causing yourself serious structural damage if you don't have perfect form in doing it, and you're not going to get it alone. Even if it's just another more-experienced member at the gym, get some help from someone who has been doing it for a while.