Be aware of the different customs before you travel. Don't offer to shake hands with someone from the opposite sex. In the cities you can dress like an American but everything will go smoother if you try to maintain modesty. Women, it is a Muslim country, you will get looked at no matter what you wear, but the small investment in a suit of Shalwar Kameez (long shirt and loose fitting pants) will go a long way towards building good relationships. Keep a long scarf (dupatta) to loop loosely around your head when appropriate.
Pay attention to how those around you respond to the call to prayer. In some areas it is best if women cover their hair at this time.
Women should avoid traveling alone if at all possible, it is not culturally appropriate in many areas.
With all those warnings, I don't mean to scare anyone away. It is a different and exciting world. Well worth the trip and very much a learning experience. Put some effort into observing the local customs and the world that tourists never see will open up around you.