There is one general rule when you are visiting someone else's home that you should follow first. Do not be in someone's house too often because they will get tired of your presence. Leave a healthy gap between visits. Meeting outside your respective houses is okay. Just make sure to keep the gaps between visits so that you are not imposing on someone's hospitality.
Now when you are in someone's house, make sure that you know where the guest of honor's seats are and make sure that you do not sit in them. The host once he sees that you are in the "humbler seat" will move you if he wants to honor you. What you do not want is for someone to make you move because you are sitting on someone else's seat.
In years past there was a rigid code that was enforced for etiquette in visiting other people's homes. Nowadays the modern etiquette is based more on common sense. Standard practice as a house guest involves bringing gifts to the host. Wine, flowers and chocolate are acceptable and safe gifts. Candles, books and other knick knacks bring up the second tier of acceptable gifts. Also always volunteer to help in ways like preparing a dish, setting up furniture or cleaning after the event. However if the host declines never insist as this would be an imposition.
If you are visiting as a house guest always leave the bathrooms, bedrooms or anything you use organized and neat. Blankets and towels should be arranged or folded. When served food eat what you are given if possible. Never try to dictate the time or the menu for meals unless it is related to a medical condition or religious belief. Leave whatever you have used in the condition that you found it. I am sure your host will appreciate it.
Although in the United States most areas are multicultural, sometimes cultural norms become crucial in your visits. Cleanliness is a general rule. Giving gifts is widely accepted but giving a gift that is too expensive are not acceptable in some Asian and South American countries like Malaysia and Peru. In Indonesia touching a person's head even a child is considered offensive. Japanese culture dictates the removal of shoes before entering a room. Be sensitive to the social norm's especially if you are in a foreign country. Better safe than sorry and increase your chances for another invite.