One more thing. Before you start the deadlift, make sure you are leaning back before you start the pull. You'll know you are in a proper position because if you let go of the bar you should fall backwards. This will ensure that your shoulders are behind the bar. Then when you start the lift pull back, not up. Shift your weight as far back on your heels as you can (If there was no weight on the bar, once again you would fall backwards) and keep the bar as close to your body as possible.
If you don't keep your weight back, the bar will travel forward and it will be much harder to maintain your arch.