No matter how much somebody loves you, being a houseguest always causes the host a certain degree of discomfort. Hosts are required to work harder and to alter their schedule to accommodate a houseguest, so it makes complete sense that you should do whatever you can to make the visit as comfortable as possible for you and the host.
One of the most important things you can do as a houseguest is to bring a present for the home you are visiting. Gifts do not have to be expensive. In fact, bringing a bouquet of flowers or some chocolates is easy and inexpensive, but it will make an impression on the host. Avoid items that are too personal, or quirky decorative items that hosts would be forced to put on display, especially if you are a first-time houseguest and don't know the family's likes and dislikes.
Once you become an official houseguest, try to make the host's life as easy as possible. Learn the home routines, such as what time breakfast is served and what time they require quiet time in the evening. Even if the schedule varies from your own significantly, you can try to adapt as much as possible. For example, if you really cannot go to sleep at the same time they do, find ways to work around the schedule so they don't have to stay up to open the door for you. Even if your offers to help are refused, you can turn into a houseguest from heaven by doing little things such as making sure you tidy up the bathroom after using or cleaning the kitchen after making some coffee.
As a houseguest, you are expected to bring luggage with you, but even so, try to make your presence as unnoticeable as possible around the house. Keep your belongings in one single place, and don't take the "make yourself at home" to mean that it is all right to leave dirty cups in the sink or your shoes scattered in the living room carpet.
Finally, when your time as a houseguest comes to end, remember to be grateful. Send a thank you note once you arrive home, and make sure your hosts know that your stay with them was a pleasure.