Okay, a proper answer. This comes from my dad having gone to Yale today!
1) They do not care about A levels. It adds to your 'points' with SATIIs, but A levels is the equivilant of the first year of undergrad degree in the US. Sad, but true. My Latin AS was the first year, A level will be the second year.
2) For Harvard and Yale, they advise a score on the SATS in the 1500's. In other words, they want nearly full scores on both tests. SATIIS you *have* to check, on collegeboard.com, as Amercian subjects are very very different. There is no history, there is 'world history' - where they look briefly at lots of countries, American history - where they go into great depth, and European history, where they study France, Germany and Britain. No real depth, actually, just breadth. American subjects cover a lot, shallowly. The best advice is to take the Literature SATII - easy if you can answer questions on grammar, and the Writing SATII, where you have to write an essay, and then an additional SATII in the closest subject to your own.
3) Personal statement for university here, throw out the window. In your personal statement essay, they want extra curricular by the wheelbarrow-load. American schools focus on it heavily - they'll want evidence of 'development' in extra curricular, ie, learning an instrument, or dance, or painting, or photography, that kind of thing.
4) You are asked to write an additional essay. It is not generally serious, I have a friend at Yale in her second year, she was asked to write on 'rain'. The general idea is to communicate yourself as friendly and happy, and enthusiastic.
5) Your teachers have to write a mini-essay on you. Choose a teacher carefully - unlike UCAS, where they write reasonably, your US application ref has to be, and I quote, 'glowing'. Exaggeration is essential, you need a teacher who can brown-nose with the best. References in America are hot stuff, and your teacher saying, 'I have enjoyed teaching him/her, but-' is not good enough for harvard/Yale/Princeton.
6) If you're into humanities, you need to brush up on science. If you're into science - it works both ways. US unis, the Ivy leagues insist on their students getting credits in both branches. You need to provide evidence that you're competent.
7) SATs are heavily coached for in the States. It's advisable to take the test twice, and order prep books online, to get used to the format. Princeton's website has a proper mock SAT to practise on online. (Princeton is the rich one)
8) If you're taking a language, England focuses more on speaking and ability than the US. Over there, you need to know grammar, especially in languages like Latin where you will have to translate into Latin from English.
9) It's expensive. Incredibly. The universities offer a 'need's blind' policy for entry, ie, if they want you, they'll shell out for your tuition, however, you also have to contribute from personal savings seperate from your parents' income, you have to submit parental income statements, and provide evidence that you work towards the cost. You're also unable to get a job over there, except on campus.
Collegeboard.com offers registration for the SATs and SATIIS, also good for locating the test centre in your area. Princeton's website - located on google, offers a free practise SAT. Fullbright commission offers practical advice and has a library in London on information, like medical insurance and stuff.