Alright, so here are my personal rules when it comes to crossing the threshold into somebody else's home, whether for a night, for a weekend, or for more:
Rule #1: Come Bearing Gifts. Never enter someone else's home without something in hand. Anything. (If you totally space on picking something up in advance, stop into a bakery, florist, or wine shop on the way over; food, flowers and alcohol may not be terribly original, but they are always appreciated.)