Change a small amount of sterling into dollars before you depart, so that you'll have some cash with you when you arrive. The general consensus, from AB contributors, is that you'll probably get the best rates from Marks & Spencer. If there's no M&S with bureau de change facilities near to you, head for your nearest post office.
Once in the USA, simply use your debit card, at ATMs, to withdraw cash. Many people open a Nationwide FlexAccount specifically for their holiday money, because Nationwide don't charge for overseas cash withdrawals using that card. (There are now several other banks which offer similar accounts). However, even if you use a card where you get charged for withdrawals, you'll still get a better deal than using a bureau de change, either here or in the USA.
Chris