I worked on a cash register with the Aloha computer program installed on it for nearly three years. I think I can give you a few tips and tricks with working on a register.
First, memorization is king in this situation. Memorize pricing, button locations, and, if you have a new register with all the whistles and bells, menus for varying options for certain items and the options themselves.
Second, it is okay to look at the register and glance up once in a while to ask a few questions. Most customers will understand this, but there are a few who will get in your face to look you in the eye.
Third, use the inability to look people in the face to an advantage. If you notice a piece of jewelry or some article of clothing, compliment the customer on it. People eat this up, and will remember it.
Fourth, try to memorize customer's orders, if appropriate. Again, people love this.
Finally, and I know you probably have heard this, the customer is always right. This means that if the customer is angry, do your best to make them happy. Where appropriate, give them something complimentary, like a coupon good for their next visit. Even if it is not your fault, apologize; people seem to love to hear this and can diffuse an explosive situation lickety split.