1. Make your arrival time known and stick to it as closely as you can. Of course if you encounter a flight delay, traffic congestion or some other unfortunate circumstance that will delay your arrival, phone your hosts as soon as possible so they aren't left anxious and wondering where you are. Make sure to pre-arrange your transportation to their home. Don't call your hosts at the last minute for a ride from the airport/train station/etc during rush hour, unless you want to make sure you're never invited back. This goes doubly for departure times. Don't make your hosts uncomfortable in their own home by staying much longer than you had agreed upon.
2. Observe how their household runs. Do they take their shoes off at the door? Do they rise early, or stay up late? Do they have a set schedule for meals? If they're vegetarians, you will want to ask if you can cook meat in their home before you assume it's alright. Do they have pets? If those pets are indoor-only, make sure you always close the door behind you. Do they compost and/or recycle? Ask them what's recyclable in their municipality so you don't leave them digging through the refuse to correct any errors. You don't have to completely change your way of being for your hosts, but to follow their lead will make the stay much more peaceful for everyone.
3. When in doubt, ask. If you are there for a short stay, I'm sure your hosts prepared for your arrival and want you to feel as comfortable as possible. If you showed up unexpectedly or if you are there for an extended time and aren't in a position to contribute to the household expenses, don't be more of a burden than necessary. This means don't eat their food without asking. If you've used the last of the bathroom tissue, replace it (either on the roll, or if that means going to the store to buy more). At the very least, a heads up to your host that, "I just used the last of the coffee, I will buy more later today," is much nicer than having them go to the cupboard in the morning to discover they can't get their caffeine fix. If you smoke and they don't, ask where you can light-up appropriately and what you should use as a receptacle. Do not leave matches or cigarette butts laying around.
4. Do things you don't necessarily want to do. You are not staying at a resort, therefore don't expect your hosts to clean up after you. I have always held myself to the rule, "leave things better than you found them." Wipe down the shower after you've used it. Don't leave toothpaste all over the sink. Take out the recycling and/or trash if you notice it's full. If the dishwasher is loaded, run it. If it's finished, unload it. Wipe off the stove if you splatter when cooking something. Also, if you say you're going to do something, do it. One long term houseguest absolutely insisted that I let her clean the bathroom. A week later, she still hadn't touched it. Then I wanted to clean the bathroom, but felt like I should ask her if it was still on her agenda. Totally awkward. You don't need to be a housemaid, but finding little ways to be helpful will keep you in good stead.
5. Find things to do with yourself. No one wants the disruption of a newly sprouted couch potato watching Real Housewives all day when they're trying to go about their normal life. Your hosts may or may not have time to show you around their city. If they can't, then be resourceful and get out of their house during the day. Especially if they work from home. If you're broke, you can always go to the local library or you can find the free days to local cultural institutions. To be under foot all the time will surely make the time pass more slowly and your hosts more eager for your departure.
6. Whether you are sleeping on the couch, the floor or in a guest room, keep it tidy. Clothes strewn around the room and on the floor isn't an appropriate way to treat someone else's space. Cups and plates should not be left in the room, nor should receipts, empty water bottles or soda cans. Remember, this is someone's home, not a hotel.
7. Be mindful of their resources. Don't leave lights on throughout the house when you've left a room, don't turn on heaters or other appliances unless you've cleared it with your hosts. No one wants you to freeze or sweat to death, but perhaps they're on a tight budget or there is a reason why something isn't being used.
8. Laundry. Ask them what their laundry schedule is. I recently had a houseguest who stayed for several weeks and, without fail, started doing load after load of laundry on Sunday. That's the day I do laundry to prepare my work clothes for the week. I mentioned it to her and she still did it again the next week. Needless to say, she doesn't have an open invitation to return.
9. Give your hosts an idea of your comings and goings. Are you going out for a late night on the town? Warn your hosts so you're not waking them up at 3am. Is there a chance you may not return for a night or two? Leave a note, email, text message or call. You don't want them to worry and they might be relieved to know they have an evening or two to themselves.
10. Don't assume you're invited to everything. Especially if you are staying for a week or more without prior planning. You hosts have a life and they may have plans that don't include you. Don't assume anything. If they haven't expressly invited you, don't make it more awkward by forcing them to explain you're not invited to their Aunt Mabel's 50th birthday party.