#1 - Be appreciative. For the most part, being a guest is a choice. Sure there are occasions when you may be obligated to appear at a function or attend an event, but for the most part if you don't want to go somewhere or do something and accept the invitation to be a guest, you don't have to do it. And if you do decide to accept an invitation, have a good attitude about it! Be grateful for whatever it is that is being done with/for you.
#2 - Ask, don't tell. It's always a nice gesture to ask your host/hostess if there is anything you can do/bring for whatever engagement. But phrase it as a question, don't simply say "Oh, I'll bring _____" and if you are told that there is really nothing you need to do or bring, respect that! If you aren't sure what to wear or what will happen, it's ok to ask, but if you don't get the answer you want, don't push it.
#3 - When in doubt, watch your host/hostess. This is a safe rule for just about anything, from what fork to use at a fancy dinner to whether it's ok to put your feet up! Also remember that, over time, things will probably relax a bit, but look to your host(ess) for guidance about when and how far that goes.
#4 - Leave it the way you found it, or better! When I was in girl scouts, one of the first rules I learned was in regards to visiting places, and you were taught to always leave a place cleaner/better than when you found it. To go above and beyond. While this may not always be possible when you are a guest, it's certainly something to try and strive for. Pick up your own trash. Keep up with your own stuff. If you move something, put it back. If you need to unplug something, plug it back in before you leave! Little things like that really do make a big, positive impression.
#5 - Share your memories. Pictures, stories, whatever. Places have a memory as well as the people who inhabit them. Share your great experiences with others, and use them to inspire you. If there's a guestbook, sign it. Pen a little thank you note. E-mail a few photos. Reminisce with your companions and your host(ess). It is good for everyone!