All my family are from over the pond so some great tips would be:
Be yourself, don't copycat anyone.....they despise pretentiousness and know-it-all Americans......so open mind is key.
As @Lagom pointed out, always be willing to buy a round. Socializing is ALWAYS done in the pubs or cafes so get ready to have a pint after work often and if you do not drink like me.....go for a pop with the boys and still buy them a round.
Do NOT claim you like a FOOTBALL club unless you really mean it, and then get ready to defend your pick!
Take the time to first explore the UNDERGROUND so that you know the train schedule and what sort of delays happen on an everyday basis. High times (busy times morning and evening rushes) in the Underground are scary to the first timer. They are incredibly packed and uncomfortable so you will learn to take the Underground early.
London is a large city but where the Michelin-star restaurants are should be centralized. Get your A-Z of London. It is a great pocketbook of maps for all of London. I used it all the time and was a God-send to me so that if there is a delay on the Underground, you know how to get around by bus or cab or even on foot with your A-Z. Buy it as soon as you get off the plane!
There are plenty of Hostels in London, the best is the ISH (International Student Hostel)in London.....it is HUGE!! St. Christopher's Hostels are okay but some can be unorganized and not as clean. There is the YHA Hostels all over the UK which are very clean and well run but more expensive. Just look up hostels in London and Hostelworld will be your guide for the most part.
Get ready to spend A LOT of money!! London is the MOST EXPENSIVE city in the world to live so get ready for amazingly shocking costs for things when you are spending in American dollars. $1000 will last you about a week and a bit there without a job so get on the job thing immediately! Working hours are less however it all depends on the place you are working. A full time job is 30 hours or more. Average wage for a cook in a kitchen in London is between 6.75-8.50 British pound an hour. There are a ton of live-in jobs so go onto The Gumtree website and that will help you find some great places.
They might razz ya a little if you are American or a colonial but they are only trying to get a rise out of you so don't get defensive and learn to laugh. The British have a wickedly sarcastic sense of humour so you will have to get used to it. If you are in London though it is so International that the odds of you working with a real British person is going to be quite rare. There will be a million different accents surrounding you so you will have to climatize yourself to their way of speaking......remember over there YOU are the one with the weird accent....lol
Most of all.....HAVE A BLAST!!
Hope some of this helps